Use a steel paperclip slide it along the screens bezel area on the left and right to locate the magnet in the display. Now fold down the display close enough so you can transfer its location on the uppercase (don’t go to far! You can damage things if you close it fully)
Now use a rare earth magnet placing it at the location you noted with the paperclip on the uppercase. Underneath the uppercase shell is a Hall sensor which senses the magnetic flux of the magnet in the lid (or rare earth magnet you are using here) as you slide it across the uppercase passing over the location the display should go off (and then into sleep) If it fails to go off then either the locate is wrong magnet in the lid not properly installed) or the sensor has failed.