According to HP it is not possible to reset an unknown firmware password and “a system board replacement is required, and additional customer costs apply”. However, according to a YouTube video there is process that works and having tested it I can confirm it does.

  1. Unplug the system.
  2. Remove the case.
  3. Remove the backup battery (towards the back left, looking from the front of the system).
  4. Remove the jumper immediately behind the power button, under the fan.
  5. Plug in the system.
  6. Press and hold the vertical button hidden behind the front panel (when the top of the case is on), about in the middle - between the USB-A and USB-C ports. This will take several minutes.
  7. Wait for the system to reboot several times. It will finally try to boot from the disk, keep holding until it gets to this point.
  8. Unplug the system.
  9. Replace jumper and battery.
  10. Plug in the system and power on normally, password is now cleared.