How can I enable YouTube to add a "click-to-buy" link on the fan-made videos to my song on iTunes?

I wrote and recorded this song nearly 10 years ago and recently put it up for sale on iTunes: http://itunes.apple.com/us/album/the-dumb-song-single/id456663996

There are many fan-made videos for the song on YouTube, a few reaching nearly half-a-million views. (Just search "The Dumb Song" and they'll come up)

How does YouTube find the songs to link on iTunes? The song went up in September, and I thought it may do it on its own. I'm not planning to make big bucks doing this, but it would be nice to let people know they can purchase the song if they like it.

Thanks!