I Use the wiring scheme of the Manufacturer , the hub powers the headlight, the headlight has a Hot+ lead and a ground- contact, that is the circuit powering the Tail light.

tape and zip tie down the 2 wire cord to the taillight , occasional spots where the heat shrink Tubing is good .

Following directions, the wire to my Brompton Taillight is taped to the rear shifter cable Then passes thru the right chainstay.