Specify user by external id

Oh, cool! Thanks.
Okay, so it looks like that endpoint is only giving me my session info. The info for the username in api_username. When I add another “username” parameter it seems to be ignored.
This sounds similar Session/current always returns api_user

Is there a better way to discover this stuff than bothering people in the forum?
docs.discourse.org looks pretty sparse, like I can’t even find that “session” endpoint when I search for it specifically.