How to log off a user when they close the window in ASP.net C#

I am using ASP.net 4.0 and C#. When I am on Development/Production and the user is logged in, when they close the window I want the session to be killed and them to be logged out. If they did not hit the log out button and then navigate back to the web page it still has them logged in. Is there a way I can reset the form authenication cookie to release it? I guess I'm asking where I should put this in my code:  FormsAuthentication.SignOut();
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