USB power does not come out MainPort or Flexi* so you must plug in flight power to power things from these.
The connector is "jst-sh 1.0" (1.0mm). Various kinds can be bought in many places.
When connecting OpLink to an FC you need jst-sh on both ends of cable, and Txd/Rxd must be crossed to connect to Rxd/Txd at the other end. Magnifying glass. Lift tiny tiny connector flap just enough to slide pin out from Txd and Rxd on one side. Plug the pins back in the other way.