Once you start the mission, you will want to proceed as normal through the first open area in to the connecting lobby towards the second open area.

Once you do that and you are in the second open area, there will be a raised platform on the right side.

Walk to the far end of this platform and you will see a black and white arrow sign on the left-hand wall. Stand on the railing right next to this arrow.

Once you are on the railing you will want to jump off and turn around, the graffiti will be on the wall directly under you.