Hi, Steph

I believe this is possible to do by customising the language pack. If you look for totara_program.php and search for the string extensionrequestmessage, you should be able to edit it here.

Note: It does contain some placeholders so be wary of that and editing this message will affect both Certifications and Programs.

I haven't tested this, so I would advise doing in development if you want to perform a quick test.

I hope that helps.

Cheers

Tom