Greetings meldabv,
Thanks for posting in the Apple Support Communities.
We understand you're looking for assistance with getting your Apple ID linked with UNiDAYS for Apple Music, right?
If we understood that correctly, let's see if the following information is able to help. For starters, have you tried changing the login email address with your UNiDAYS to see if that will allow you to link this correctly? If not, you may be able to do this within the account settings when you're logged in. We were able to find this information in the following resource from UNiDAYS Support: How do I change my login email address? - UNiDAYS Support
How do I change my login email address?
You can change your login email address from your account settings.
If that doesn't help, contact Apple Support directly to check into this with you in more detail. This may need to be handled by UNiDAYS; however, you can verify this first by reaching out directly.
Have a great day!