Story related. If you want more info, continue reading. Otherwise, know that this will unlock through the natural progression of the story.
When you assume control of Batman, you’ll have a couple of brawls to get through before you follow a blood trail to the entrance of Harley’s base. You’ll be told to interrogate three other thugs that know the code to get in. All will be marked on the map. Just be careful with not taking down the interrogation targets.
Target #1 – At ground level with four other thugs. None are armed.
Target #2 – On a roof with three other thugs. All are armed with machine guns.
Target #3 – On a roof alone, but with a machine gun. There are also three snipers targeting that rooftop, so take them out first.
Once you enter the hideout, this will unlock.