When I tried to register here: https://www.magix.com/gb/support/my-service-center/

I got this:

Serial number already used

The serial number you entered has already been used. You may have already registered. Registering again is not necessary. If you have any questions, please contact out technical support team: http://support.magix.net
I'm using a trial version, but don't see anywhere I can input the serial number. Do I need to reinstall it?