You need to implement a new login module copying the existing one, and at the top of login event just check cookie and do FormsAuthentication.SetAuthenticationCookie(username) and you are done!
You need to re-think in terms of security and make sure you want to do it. Instead of using cookie, you can do a url login where you have to do following:
Generate a url like /yourpage.aspx?user={bacd}&signature={hashcode of secret+username}
In yourpage.aspx you need to regenerate the signature using secret+username and match it, if it matches, just do FormsAuthentication.SetAuthenticationCookie(username) and redirect to appropriate page.
Hope this helps.