HP LaserJet Print to Mail Accessories - Manual (Page 13)

EN Installing the accessory 11
7 Locate the accessory interface cable (C-Link) and plug it into the
connector on the printer labeled “ACCESSORY.”
8 Plug the power cable into the power connector on the front,
toward the right, of the accessory.
9 Route the accessory power cable, printer power cable, and
printer data cable through the rectangular cutout in the left side of
the reversible base.
Figure 6. Connecting accessory cables