Hello,
If you are adding new accounts in plesk and want your customers to add "default domain administrator permissions", then, you'll need to set permissions for your client accounts to edit/set "Domain Administrator".
Once your client logs in
Plesk Control Panel using the login details you have provided, they will be able to edit the "Domain Administrator" permissions.
If you still face any confusion in this, please get back to us using our 24x7 live chat support to get the issue resolved