The request to register a generated access grant should go to the Auth service, then you will use S3 credentials with the Gateway-MT.
See also
and
We use this Community forum and the Helpdesk portal https://supportdcs.storj.io, however to have a personalized support on Helpdesk you must be an owner of the PRO account (you should have a payment method in your account).
You may also use this page to make a request for the partnership: The Storj Partner Program.