Hi, this should be enough.
<script type='text/javascript'>
<!--
function ExitPop() {
return "*****************************************************\n\nWAIT! -- \n \n \n XXXX XXXX XXXX XXXX\n XXXX XXXX XXXX XXXX\n XXXX XXXX XXXX XXXX\n XXXX XXXX XXXX XXXX\n XXXXXXX XXXXXXX\n XXXXX XXXXX\n XXXXXXX XXXXXXX\n XXXX XXXX XXXX XXXX\n XXXX XXXX XXXX XXXX\n XXXX XXXX XXXX XXXX\n XXXX XXXX XXXX XXXX\n \n \n \n -- Click the CANCEL button Now!\n\n*****************************************************"
}
// -->
</script>
<SCRIPT LANGUAGE = "JavaScript">
<!--
window.onbeforeunload = ExitPop;
// -->
</SCRIPT>
By the way, using the onbeforeunload event is not really nice (and I'm not really sure if it's compatible across all the browsers). If you can, you should avoid using it.