Yeah, the JSON receiver has two receive types, one for legacy support and one for the more intuitive way it works. The legacy (Parameter receive type) use is presented here: https://forum.infiniteautomation.com/topic/2642/how-to-use-http-json-receiever-to-receive-the-data-with-is-by-json-format

For the more intuitive format, where the whole POST body is JSON for instance and the content type is application/json, then the "HTTP parameter name" field needs to be a JSON pointer. Someone linked in this thread to a tool to help craft JSON pointers: https://forum.infiniteautomation.com/topic/3029/json-retriever-documentation-missing/5

But in short it's like a path, like
/ on an alphanumeric point should catch the whole JSON object.
/key should get whatever is at {"key":...}
/keyToArray/5 should get the fifth index of the array at {"keyToArray":[...]}