@josias I put the message shown in the pic in a div set to display:none. Then I load a Javascript file which I've named popunder.js. That JavaScript file adds an empty div to the page. Finally, once the page has loaded, some more JavaScript checks for the presence of the div added by popunder.js, and if not found, shows the congratulations message.
If the visitor has JavaScript disabled they get a different congratulations message that pats them on the back even more.