We want to use our maindomain.com as the main entrance, and after login the user should be able to visit maindomain.dk, support.maindomain.com or similar domains/subdomains

Whats the best way to keep the user login status across multiple domains/subdomains?

Everything runs from the same Umbraco Instance.

It works by linking from maindomain.com to maindomain.dk?auth=<authenticationcookievalue>, and then setting a cookie on the second domain but I'm not sure if that is secure enough?