How to redirect to the default by opening a new window in form authentication?

Hi, I've got a problem with opening a new window to display the defaultUrl.  My application has a login using form authentication, once user clicks the login button, it redirect the user to the defaultUrl within the same window, I wants to open another new window for displaying defaultUrl, how can i do it?

 
 <authentication mode="Forms">
            <forms name="fortress"
             loginUrl ="login.aspx"
             defaultUrl ="edit.aspx"
             protection ="All"
             timeout ="60">  

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