Do I also need to be assigned as a delegated administrator/CSP reseller?

Being assigned as a delegated administrator/CSP reseller of the customer enables partner to manage customer effectively with a single sign on through Microsoft Partner Center. It is highly recommended for reseller to be assigned as a delegated administrator or CSP reseller of record by requesting Reseller relationship through Microsoft Partner Center.

More details about how to request a CSP reseller relationship can be found here.