Create a forced advertisement on button click for website.

I have a website where I give away free downloads. Currently the users can simply and easily click the download button the the audio page and the download starts. I want use any available ad solution that allows me to force the visitor to watch a 15 -20 second pop up video ad whenever the click the download button. I have seen this done a few times on Facebook applications especially with games. The user is forced to watch a new ad each time they start a new game. Although I know its being done, I have not found any solutions that offer it so I am hoping freelancers can help. I am happy to sign up for whatever add serving system is need to finish the project ( ie double click etc.) I currently have an active ad sense acct. and will use whatever is needed. If you have read this far let me know by starting your proposal with 'forced ad.' I will be adding many new files to the website So i need to be able to include this code to each new download button i create. Thanks for your feedback.