how to authenticate user after prompting message?

I have a username, password and btnLogin , when user hits the btnLogin after entering password and username, I go check the see if user exists in my UserTable in database if exists then DisplayPanel pops up and shows messages like so many days to left for your membership to expires etc.. That DisplayPanel has OK button , which user reads to message and hits OK button to continue, that moment I am authenticating user to go on... How can I achieve this with C# code for using  FormsAuthentication.RedirectFromLoginPage ?
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