Warning: Optional features are subject to change.
Activating optional features for GitHub Apps
-  In the upper-right corner of any page, click your profile photo, then click Settings.  
-  In the left sidebar, click Developer settings.  
-  Select the GitHub App you want to enable an optional feature for. 
-  In the left sidebar, click Optional Features.   
-  Next to the optional feature you want to enable for your app, click Opt-in.  
Activating optional features for OAuth Apps
-  In the upper-right corner of any page, click your profile photo, then click Settings.  
-  In the left sidebar, click Developer settings.  
-  In the left sidebar, click OAuth Apps.  
-  In the left sidebar, click Optional Features.   
-  Next to the optional feature you want to enable for your app, click Opt-in. 