If you want to manage a customer's service or subscription on their behalf, the customer must grant you administrator permissions for that service or subscription and sign the Microsoft Customer Agreement.
If you want to establish a reseller relationship with a customer and manage only the Azure subscriptions that you provision, it is not necessary to obtain administrator permissions. For more info please read here: https://docs.microsoft.com/en-us/partner-center/request-a-relationship-with-a-customer