If you look at this reply I just sent you, you'll see at the top it says "
HtBlack posted a REPLY on ...". If you hover on the word "reply" you'll see it's a link, click on it and it will take to you to the comment/reply page, and you just copy the URL of that page and put it in your Artist's comments specifying it's the link to the critique you gave.
If it's too complicated, you can give a link to the piece you wrote a critique on, and we can look for it in the comments. (: