Just add your username and the icon should link automatically as a youtube.com/user/[name].

newer youtube accounts don't get a username link until they have a certain number of subscribers (100 I think). I had to put channel/UCJmX2au1-NmxWTLOrxUWZSQ in the box (from my channel page) to get it to work, so try your equivalent of that to see if it works.

Trust YouTube to change something that was completely fine. Absolute mongs.