How to use a Shortcut in iOS to access the API

I'd like to be able to update the inventory across some disjointed applications that don't talk to each other, but I'm not sure where to find the API reference for using a key or a login.

Ideally, I've used API keys as part of a URL, and then added data headers. I'm not sure where to look. I've gone through a ton of the API documentation and the overviews and haven't seen how to do it.