Javascript alert message on browser close and redirect

Hello WPMU.

I have a client who would like on her sales pages that when a user wants to close her web page that a message would appear saying

"Wait! Have you claimed Your FREE CD Yet?"

And then if the visitor chooses something like "Stay on This Page" then the user would be redirected to the url which has the free cd offer.

I've had some hours of searching and stackoverflow reading and have the following...

var hook = true;
      window.onbeforeunload = function() {
        if (hook) {
          return "Wait! Have you claimed Your FREE CD Yet?"
        }
      }
      function unhook() {
        hook=false;
      }

Everything from there works fine, but the redirect url is what I can not find how to work so far anywhere on the internet with a solid explanation of how to do this.

Also, I was wondering if this feature is included in OptimizePress? That is pretty much the only place I recall seeing this implemented.

How can I achieve this?

Best regards WPMU!