Javascript using the OnUnload Event

Hello,

I have created an E-learning Course using captivate, at the end of the course I want the users to give me a feedback,

So I have to write a javascript code which will execute whenever the user clicks on the close button of the browser

I have used the onunload function and added it to the slide on enter.

onUnload=window.open('http://www.google.com');

onbeforeunload=window.open('http://www.google.com');

This does work but not when the users click on the close button.

Can anyone help me with this requirement.