Any negative will work.
The little leg on the MCU that controls the switch has a small positive “potential” on it. This needs to touch ground to complete the circuit and signal the MCU. You can run the positive wire from this leg or pad to the switch and then the return wire can go to any ground point you want. It can be on the driver or the flashlight body. There are many negative points on the driver, maybe even a dedicated pad for it.
PS, this is an old thread. New questions should go to the new Batch 7 thread.