AMP doesn’t use API keys as far as I know. You need to login via the API (API/Core/Login) to get a session ID first, then add that to the body as SESSIONID.
I got a bunch of notes in this repo here: https://github.com/p0t4t0sandwich/ampapi-spec
And a pin with more info in the development channel of the discord.
The Session.Exists error just means you didn’t authenticate correctly.
Note: Core/GetStatus also doesn’t take in any parameters, since you call it from the instance’s web endpoint (or proxy the API instance through the ADS)