To register as a domain administrator you must use the postmaster@YOURdomaintoadmin.com email address associated to the domain you are wish to manage. Therefore you must have access to emails sent to the postmaster email address for the domain you wish to administer. Once you have access to emails send to postmaster@ simply register here
and click on the registration confirmation in the confirmation email.
For domains that use Google GSuite or Microsoft Office 365 we have included a video for each application showing how to access postmaster email addresses.