HP LaserJet Print to Mail Accessories - Manual (Page 14)

12 2 Setup EN
10 Slide the accessory toward the rear of the printer so that the
alignment guide from the accessory slides firmly into the duplexer
slot.
Figure 7. Attaching the HP LaserJet Print to Mail accessory
11 When the tips on the bottom of the alignment guide rest in the
indentations in the duplexer rails, the front of the accessory is
approximately one-eighth of an inch (3.18 mm) from the rear of
the printer.
12 Flip down the rear output door of the printer into the opening in
the accessory until it stops. The top edge of the rear output door
should be even with the paper feed opening of the accessory.
Note When the rear output door of the printer is flipped down, the printer
delivers printed sheets to the HP LaserJet Print to Mail accessory. To
deliver printed sheets to the printer’s top output bin, simply flip the rear
output door back up to its closed position.
Top output bin
Rear output door