The Microsoft account that you need to register for the customer account must be connected with your CSP partner account. You can check whether this is already done by using the method described in Checking the Status of the Microsoft Account.
If the Microsoft account is not connected with your CSP partner account, request the owner of that Microsoft account to connect it by opening an invitation page and accepting your invitation. To obtain the contact data of the owner and the URL of the invitation page, use the method described in Checking the Status of the Microsoft Account.