FB Comment Guards
Comment Guard allows you to set up an opt-in message for anyone that leaves a Comment on your Facebook Posts, Boosted Posts, or Ads.
Setting up a comment guard on a post or ad will allow your bot to automatically send a message to anyone who comments on that post. This is an incredibly easy way to grow your Messenger opt-in list.
NOTE: Comment guards do not work on events. If you share an event URL on your page, Facebook will convert it into an event.