How to extend Mural with custom menu option?

Hi everyone,

When I was looking at the GitHub integration I noticed that it is apparently possible to extend Mural with a custom menu option, but I cannot seem to find it in the developer website.
In the GitHub integration the menu in called "Send to GitHub" which is offered when you select sticky notes and you open the context menu (Right click).
Have I missed an important page with more developer information?
Thanks for helping out 🙏