Help returning a value from a hidden field withing a usercontrol

Could someone help with an unusual request.  I have a usercontrol called 'ToolBar' with an ImageButton named "ibMail" and an OnClientClick event that calls OnClientClick="PopUpMail('Mail.aspx');"  here is the javascript fuction within the usercontrol.

    function PopUpMail(URL) {         /* Note: I add the datetime to make the url unique.  If you don't do this         the window gets cashed and page load will not fire the second&nb ...