During Mission 6: Return to the Tower, you will travel back to Dunwall Tower to kill Lord Regent. Upon arriving there Samuel will inform you about the broadcast tower that all of the propaganda is aired on, opening up an optional mission to travel to it and investigate.
After making your way through the heavily guarded yard you will finally reach the mansion. If you entered through the frontmost method you will find yourself in the main lobby, as shown in the video. Once you do use Blink to ascend the chandeliers and sneak into the Lord Regent’s chamber. You will see a safe to your right; the combination is 935. Open it and take out Lord Regent’s confession audiograph.
Exit through the balcony and Blink towards the left hallway. Turn right and travel down the hallway until you see the Arc Pylon. If you have Bend Time 2 use it to walk past easily, otherwise use Blink to travel just above it onto the stairs. Walk up the stairs and activate the broadcast system to unlock the trophy.
This is also the fourth and final target you must neutralize through indirect means for the Poetic Justice trophy. However, it will not unlock until after you have returned to Samuel and completed the mission.
To see a walkthrough of the the method being performed, please see the video below: