This style of buzzer has remote video and a handset for sound, so the "buzz-in button" isn't directly connected to the door lock. The only external attachment is a header cable to board which connects the video, audio and door buzzer signals in series to other apartments.
Finding the door lock wires was a bit challenging. The model numbers listed on the parts led to some very vague schematics which suggested the door lock circuit would be a 12V supply which switched to ground to open the lock. This means the button was an NO or Normally Open style, so we'll see next we connect the ground and the 12V door open signal to the NO terminals on the wireless relay board. If you have trouble finding schematics, use a multimeter to find likely candidates and carefully play the guessing game by jumpering the terminals until the door opens. Make sure any circuit you close is between 5V and 24V DC to prevent a nasty surprise.
I used the multimeter to find a 12V signal and determined the yellow wire was the bingo, and obviously enough, the black wire was ground.
Wireless Relay Board
The relay board goes into the closet behind the door buzzer housing and is connected to the buzzer by fishing the wires through a small hole drilled in the back of the wall.