Passing a form variable from the login page to the protected php file

I need to pass a form value from the amember login page to the protected page that it redirects to. I will be retrieving the value via:

$Challenge=$_REQUEST["challenge"];

line in the login.php file. I need to continue to pass this value to the protected page - any ideas on how can I pass it as a hidden form variable (or session variable) to the php file that the user is redirected to after logging in?

Thanks.