Mods to move property setting into service class.

src/main/java/org/gitlab4j/api/services/ExternalWikiService.java

 package org.gitlab4j.api.services;
 
+import org.gitlab4j.api.GitLabApiForm;
+
 public class ExternalWikiService extends NotificationService {
 
     public static final String WIKIURL_KEY_PROP = "external_wiki_url";
 
+    /**
+     * Get the form data for this service based on it's properties.
+     *
+     * @return the form data for this service based on it's properties
+     */
+    @Override
+    public GitLabApiForm servicePropertiesForm() {
+        GitLabApiForm formData = new GitLabApiForm()
+                .withParam("external_wiki_url", getExternalWikiUrl(), true);
+        return formData;
+    }
+
     public String getExternalWikiUrl() {
         return this.getProperty(WIKIURL_KEY_PROP);
     }

src/main/java/org/gitlab4j/api/services/HipChatService.java

 package org.gitlab4j.api.services;
 
+import org.gitlab4j.api.GitLabApiForm;
+
 import com.fasterxml.jackson.annotation.JsonIgnore;
 
 public class HipChatService extends NotificationService {
@@ -11,7 +13,32 @@ public class HipChatService extends NotificationService {
     public static final String API_VERSION_PROP = "api_version";
     public static final String SERVER_PROP = "server";
     public static final String NOTIFY_ONLY_BROKEN_PIPELINES_PROP = "notify_only_broken_pipelines";
-    
+
+    /**
+     * Get the form data for this service based on it's properties.
+     *
+     * @return the form data for this service based on it's properties
+     */
+    @Override
+    public GitLabApiForm servicePropertiesForm() {
+        GitLabApiForm formData = new GitLabApiForm()
+            .withParam("push_events", getPushEvents())
+            .withParam("issues_events", getIssuesEvents())
+            .withParam("confidential_issues_events", getConfidentialIssuesEvents())
+            .withParam("merge_requests_events", getMergeRequestsEvents())
+            .withParam("tag_push_events", getTagPushEvents())
+            .withParam("note_events", getNoteEvents())
+            .withParam("confidential_note_events", getConfidentialNoteEvents())
+            .withParam("pipeline_events", getPipelineEvents())
+            .withParam("token", getToken(), true)
+            .withParam("color", getColor())
+            .withParam("notify", getNotify())
+            .withParam("room", getRoom())
+            .withParam("api_version", getApiVersion())
+            .withParam("server", getServer())
+            .withParam("notify_only_broken_pipelines", getNotifyOnlyBrokenPipelines());
+        return formData;
+    }
 
     public HipChatService withPushEvents(Boolean pushEvents) {
         return withPushEvents(pushEvents, this);

src/main/java/org/gitlab4j/api/services/JiraService.java

@@ -2,6 +2,8 @@ package org.gitlab4j.api.services;
 
 import java.util.Map;
 
+import org.gitlab4j.api.GitLabApiForm;
+
 import com.fasterxml.jackson.annotation.JsonIgnore;
 
 public class JiraService extends NotificationService {
@@ -14,6 +16,25 @@ public class JiraService extends NotificationService {
 
     private CharSequence password;
 
+    /**
+     * Get the form data for this service based on it's properties.
+     *
+     * @return the form data for this service based on it's properties
+     */
+    @Override
+    public GitLabApiForm servicePropertiesForm() {
+        GitLabApiForm formData = new GitLabApiForm()
+            .withParam("merge_requests_events", getMergeRequestsEvents())
+            .withParam("commit_events", getCommitEvents())
+            .withParam("url", getUrl(), true)
+            .withParam("api_url", getApiUrl())
+            .withParam("project_key", getProjectKey())
+            .withParam("username", getUsername(), true)
+            .withParam("password", getPassword(), true)
+            .withParam("jira_issue_transition_id", getJiraIssueTransitionId());
+        return formData;
+    }
+
     public JiraService withCommitEvents(Boolean commitEvents) {
         return withCommitEvents(commitEvents, this);
     }

src/main/java/org/gitlab4j/api/services/MattermostService.java

 package org.gitlab4j.api.services;
 
+import org.gitlab4j.api.GitLabApiForm;
+
 import com.fasterxml.jackson.annotation.JsonIgnore;
 
 public class MattermostService extends NotificationService {
 
     private String defaultChannel;
 
+    /**
+     * Get the form data for this service based on it's properties.
+     *
+     * @return the form data for this service based on it's properties
+     */
+    @Override
+    public GitLabApiForm servicePropertiesForm() {
+        GitLabApiForm formData = new GitLabApiForm()
+            .withParam("webhook", getWebhook(), true)
+            .withParam("username", getUsername())
+            .withParam("channel", getDefaultChannel())
+            .withParam("notify_only_broken_pipelines", getNotifyOnlyBrokenPipelines())
+            .withParam("notify_only_default_branch", getNotifyOnlyDefaultBranch())
+            .withParam("push_events", getPushEvents())
+            .withParam("issues_events", getIssuesEvents())
+            .withParam("confidential_issues_events", getConfidentialIssuesEvents())
+            .withParam("merge_requests_events", getMergeRequestsEvents())
+            .withParam("tag_push_events", getTagPushEvents())
+            .withParam("note_events", getNoteEvents())
+            .withParam("confidential_note_events", getConfidentialNoteEvents())
+            .withParam("pipeline_events", getPipelineEvents())
+            .withParam("wiki_page_events", getWikiPageEvents())
+            .withParam("push_channel", getPushChannel())
+            .withParam("issue_channel", getIssueChannel())
+            .withParam("confidential_issue_channel", getConfidentialIssueChannel())
+            .withParam("merge_request_channel", getMergeRequestChannel())
+            .withParam("note_channel", getNoteChannel())
+            .withParam("confidential_note_channel", getConfidentialNoteChannel())
+            .withParam("tag_push_channel", getTagPushChannel())
+            .withParam("pipeline_channel", getPipelineChannel())
+            .withParam("wiki_page_channel", getWikiPageChannel());
+        return formData;
+    }
+
     public MattermostService withPushEvents(Boolean pushEvents) {
         return withPushEvents(pushEvents, this);
     }

src/main/java/org/gitlab4j/api/services/NotificationService.java

@@ -4,12 +4,12 @@ import java.util.Date;
 import java.util.HashMap;
 import java.util.Map;
 
+import org.gitlab4j.api.GitLabApiForm;
+
 import com.fasterxml.jackson.annotation.JsonIgnore;
 
 public abstract class NotificationService {
 
-    public static final String WEBHOOK_PROP = "webhook";
-    public static final String USERNAME_PROP = "username";
     public static final String NOTIFY_ONLY_BROKEN_PIPELINES_PROP = "notify_only_broken_pipelines";
     public static final String NOTIFY_ONLY_DEFAULT_BRANCH_PROP = "notify_only_default_branch";
     public static final String PUSH_CHANNEL_PROP = "push_channel";
@@ -22,6 +22,15 @@ public abstract class NotificationService {
     public static final String PIPELINE_CHANNEL_PROP = "pipeline_channel";
     public static final String WIKI_PAGE_CHANNEL_PROP = "wiki_page_channel";
 
+    public static final String WEBHOOK_PROP = "webhook";
+    public static final String USERNAME_PROP = "username";
+    public static final String DESCRIPTION_PROP = "description";
+    public static final String TITLE_PROP = "title";
+    public static final String NEW_ISSUE_URL_PROP = "new_issue_url";
+    public static final String ISSUES_URL_PROP = "issues_url";
+    public static final String PROJECT_URL_PROP = "project_url";
+    public static final String PUSH_EVENTS_PROP = "push_events";
+
     private Integer id;
     private String title;
     private Date createdAt;
@@ -42,6 +51,8 @@ public abstract class NotificationService {
 
     private Map<String, Object> properties;
 
+    public abstract GitLabApiForm servicePropertiesForm();
+
     public Integer getId() {
         return id;
     }

src/main/java/org/gitlab4j/api/services/SlackService.java

 package org.gitlab4j.api.services;
 
+import org.gitlab4j.api.GitLabApiForm;
+
 import com.fasterxml.jackson.annotation.JsonIgnore;
 
 public class SlackService extends NotificationService {
     
     private String defaultChannel;
 
+    /**
+     * Get the form data for this service based on it's properties.
+     *
+     * @return the form data for this service based on it's properties
+     */
+    @Override
+    public GitLabApiForm servicePropertiesForm() {
+        GitLabApiForm formData = new GitLabApiForm()
+            .withParam("webhook", getWebhook(), true)
+            .withParam("username", getUsername())
+            .withParam("channel", getDefaultChannel())
+            .withParam("notify_only_broken_pipelines", getNotifyOnlyBrokenPipelines())
+            .withParam("notify_only_default_branch", getNotifyOnlyDefaultBranch())
+            .withParam("push_events", getPushEvents())
+            .withParam("issues_events", getIssuesEvents())
+            .withParam("confidential_issues_events", getConfidentialIssuesEvents())
+            .withParam("merge_requests_events", getMergeRequestsEvents())
+            .withParam("tag_push_events", getTagPushEvents())
+            .withParam("note_events", getNoteEvents())
+            .withParam("confidential_note_events", getConfidentialNoteEvents())
+            .withParam("pipeline_events", getPipelineEvents())
+            .withParam("wiki_page_events", getWikiPageEvents())
+            .withParam("push_channel", getPushChannel())
+            .withParam("issue_channel", getIssueChannel())
+            .withParam("confidential_issue_channel", getConfidentialIssueChannel())
+            .withParam("merge_request_channel", getMergeRequestChannel())
+            .withParam("note_channel", getNoteChannel())
+            .withParam("confidential_note_channel", getConfidentialNoteChannel())
+            .withParam("tag_push_channel", getTagPushChannel())
+            .withParam("pipeline_channel", getPipelineChannel())
+            .withParam("wiki_page_channel", getWikiPageChannel());
+        return formData;
+    }
+
     public SlackService withPushEvents(Boolean pushEvents) {
         return withPushEvents(pushEvents, this);
     }