The REST API docs are an installable option for openHAB.
Then appears as an option on your UI landing page, you use it interactively for try-it-and-see.
Once you’ve worked out the URL, you can analyze the returned JSON in your rule, perhaps using the JSONPATH installable service.
You don’t generally need to do any of this in normal home automation use, so it’s not particularly easy.
Yes I do have Rest API installed, used JSONPATH a bit while stearing my mower with it until someone created a lovely binding. Currently I stuck to get the http string (which I’ve got from Rest API) called from a simple rule to have the JSON in an item to use it further :-/
I’m running all this on a Windows System, and currently try to get JSON information wih provided http-String I have got from Rest API from requested UID.