on leave page ask to go somewhere else

I am using this code to alert the user of an admin page that there is one more thing to do, 'does he want to go to that page now?' confirm/cancel.

function CheckLeave(){
	if(window.event) {
		window.event.returnValue = "Would you like to mark this vehicle as sold?";
		parent.location="admin2.asp?linkID=<%=thisID%>";
	}
}

Code (markup):

in the body tag I have:

on onbeforeunload="CheckLeave()"

Code (markup):

which is fine for this purpose.

My problem is am not sure how to do one thing if he confirms and another if he cancels. ??