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:
“C:\Program Files (x86)\Parallels\Plesk\admin\bin\panel_gui.exe" -p -domain_registration_url <url>
To change the URL of the Manage Domain Names button, run the following command:
“C:\Program Files (x86)\Parallels\Plesk\admin\bin\panel_gui.exe" -p -domain_management_url <url>