What api? What function would it serve on your forum (be specific)? Are we just showing a live update feed of some coin prices, are we using it to somehow pull their info from some other website using a username they put in a custom profile field to be able to use somehow get that info from a provided json? I can come up with all sorts of ideas but it sounds as if you're contemplating this rather than actually wanting to implement something that exists.
I am a web developer/administrator, specializing in forums. If you have work you need done or are too lazy to do, pm me!