In Chapter 3, the room becomes accessible after completing all of Physical Alice's missions. Go to Level P and head to the area where Striker first spawns. In the back corner is a wall with a Boris poster, behind which is the room. Additionally, entering the room will not protect Henry from Ink Bendy.
In Chapter 4, the room becomes accessible when Henry acquires the turnwheel and opens the door to the archives. Once the door opens, go back toward Grant Cohen's office and open the door opposite it, revealing a small closet. Walk through the back wall of the closet with a Boris poster to enter the room. This is the only time the room appears at the beginning of a chapter.