If you are asking how to make a page public, in Kentico there are settings both on the pages (Security) that you just need to make sure do not require authentication, and also on a site-level (through the Settings).
If you are trying to make it so a user (through a link) can be automatically logged in and then view a login-in-required-page, then you will need to implement a custom web part that somehow validates a user through the url (maybe some temporary url parameter) and uses the below code to log them in through the API:
CMSContext.AuthenticateUser("TheUsername", true);