Add-On Installation from Linked GitHub repo

I recently submitted an Add-On on the File Exchange as a link to a GitHub repo. My GitHub repo is setup to be packaged as a toolbox (which I have done so and have an .mltbx file as a release on the GitHub page).

I was expecting to be able to go in the Add-ons Explorer and install it as an Add-On but it only list "Download" as an option.

How do I set up my GitHub repo so that the toolbox is available to install as an Add-On?

[Jan Kappen]

And here is the follow-up: How to use toolboxes without publishing it to File Exchange? We need to use closed source, internal packages in our company.