Are you talking about .be domains ? You cannot just push a .be (or most country TLDs) like .com. Even if the registrar supports an internal account change (push), this will effectively transfer control of the domain name but not ownership.
The only way to change registrant is to use the registry's trade procedure (which involves a small cost).
Registrar's users have some kind of unique customer code or usernames as they said above
There is a procedure where you state the domain you want to push and the details of the new owner. To be 100% contact your registrar because you can't push back the domain if you make an error