Stop Clients Login Portal

My Mautic version is: 2.15.3
My PHP version is: 7.1.32-1+0~20190902.23+debian9~1.gbp9d1be7

Your problem
My problem is:
When I send emails to clients the custom domain m.demo.uk/t13412 they can access my login portal by cutting off m.demo.uk/ which takes anyone to my login portal.

Any way to get past this?

These errors are showing in the log: None

Steps I have tried to fix the problem: Searched the forum