Reading instructions isnt hard...
In the drop down under your name click Personal Details...
Scroll to Youtube Channel... now if you read it carefully, it will tell you what goes in the box...
Just incase you didnt see it, this is what it says
Just your YouTube Channel ID, not the full link. For example, enter "UCGUBZrH31AkJK-_JZxMLOKQ" and not "
http://www.youtube.com/user/mcnfreedom" (find your custom ID
Here )