Story related. If you want more info, continue reading. Otherwise, know that this will unlock through the natural progression of the story.
This will unlock after the very last fight in the DLC. You’ll have to fight some robots (yeah, robots) in the room with the large Joker shrine. The only difficult thing with them is that it won’t show when they’re about to hit you, so have to pay attention to the ones that you’re not attacking. Harley will also fire grenades down at you. Once they’re all destroyed, go up to Harley, counter, and watch.