How do API things work with ROBLOX? Like a Trello application center

There is little to no documentation on how to integrate an offsite service to ROBLOX with an API. I've seen it done with application centers that go to Trello and some other websites, but I have no idea how it's done. Is there any documentation that I am missing?

(sorry this is kinda a request, lol)

I believe you are looking for HttpService. This can allow you to interact between your ROBLOX game and other web servers.

Hope this helps :)