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 https://docs.microsoft.com/en-us/partner-center/request-a-relationship-with-a-customer