New-RdsTenant : User is not authorized to query the management service.

Hey StefanKi, 

for this to work you have to make sure the current user is assigned as a TenantCreator in the "Windows Virtual Desktop" application (in 'Users and groups' blade).

Also, once you did that, wait at least 4-5 minutes before proceeding to log-in and try to create the tenant. If you do that before, invoke Add-RdsAccount again, to re-login.

I hope that helps.

Sincerely,

Alex