It does indeed have a J18 port as seen in this picture circled in red that I just took inside my case. Keep in mind that I am using my OEM case and OEM motherboard, and I've had this computer for two years.
have you tried following the oem case wires. the mb maker was foxcon and a lot of the photos online show pin on the j17 port. if there gone then there has to be another port that hp used for power on and reset.