The API really isn’t going to be documented for your use case as it’s totally unsupported (I do have at least one client who’s doing something similar, though). How to reverse engineer the Discourse API is probably the best you can get and also https://github.com/discourse/discourse/blob/master/config/routes.rb.
If you’re using an admin API key, then all restrictions are removed, so you should be able to create the topic. Or perhaps create the topic as the user then move it to the category that you don’t want them to be able to post in?