To set this flag, just pretend you were going to add a comment to this (or any other post) and "Click the 'Mark interesting' button to monitor new comments on your 'View your activity' page."
I'll be adding a "Please click the mark interesting button" tag to other posts that I want you to follow.
I think the flag is set automagically when you add a comment.
Keywords: 2/19, 80%, email, Please click the mark interesting button