Manage session timeout in custom page

Hi. When the user is in a custom page, the session timeout warning box is not displayed, so when he finishes longer works and press “SEND” (eg if the custom page is a complex form), he is banned out and looses its data.
How can I alert the user for finishing session timer in the custom pages like the others?
Thanx

Make sure that you have enabled the “Include common files” option for the custom files.