You have to jump on the big "button". There's two white rectangles("buttons") on the ground, and one is already activated, the other is pressable. If you already did that, you have to press shift or left-click while on ground to go under the room.
Oh, I just realized, I had a force field between the two buttons, but it seems to have disappeared in the build. Basically, you're not allowed to go left, you have to press shift or left-click.
