IsAuthenticated status change?

Without using the standard login controls to take care of the user authentication and login of a user, how do I change a specific user's "IsAuthenticated" status to "true"?

I have tried to set "User.Identity.IsAuthenticated = true", but I get an error stating that I can not assign a value to "User.Identity.IsAuthenticated" as this is read only. So basically I want to log a user in on the fly by just getting the username.

Thanks!

Regards

Jan
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