Hi, Suzanne,
As I understand it, you can't add this to your own video. These are added when a video with unauthorized music is uploaded to YouTube. Many record labels have a deal with YouTube to add this link, instead of just muting or not allowing the video to air.
So, when you upload a video with music, one of several things will happen:
1. You're using the music legally, in which case there's no problem.
2. You're using the music without permission, in which case the video is declined or the audio track is removed (muted).
3. YouTube has permission to allow the music, as long as a purchase link is provided.
You might do better to scour the web for royalty-free music, which is what I've done. You can even search YouTube for informational videos on finding and using royalty free music; that way, the copyright issues are moot.
Hope this helps!
My YouTube channel: http://www.youtube.com/user/art166wj