The provided link was in the form:
After investigating the validity of the flag, I clicked the "flag or disagree" button and entered a custom message that went something like this:
"Dupe answer: http://askubuntu.com/a/256822"
However, I received an error stating that a post ID or link was required to flag as a duplicate (I forgot to get a screenshot of the error). It seems like the short URL for answers is not recognized.