Hello AlB

Please go to this location and copy the file contents:

plugins/system/helixultimate/overrides/com_users/registration/default.php

Then paste it here:

templates/shaper_helixultimate/html/com_users/registration/default.php

If you do not find the file in templates folder, then create it. Then you can edit the file to add more fields to your registration form.

Best regards