You can change the domain name registrar's URLs only through command line.
To change the URL of the Register Domain Names button, run the following command:
/usr/local/psa/bin/panel_gui -p -domain_registration_url <url>
To change the URL of the Manage Domain Names button, run the following command:
/usr/local/psa/bin/panel_gui -p -domain_management_url <url>