Geocortex Reporting REST API with On-Premises Installation?

Does anyone know if it is possible to leverage the REST API to run reports with an on-premises installation? Support "believes it is possible", but couldn't give specifics.

I am getting a 403: invalid portal URL response that I can't seem to get around when attempting to obtain the run token from /auth/token/run. I am a self-taught beginner when it comes to APIs and web requests, so it has been all trial and error up to this point, and I may be missing something obvious. Any direction here would be appreciated.

Thank you,