The wireless unit D-Fly has a plug on each end. I hung mine below the seatpost mounted Di2 battery.
I coiled the cord between the battery and D-Fly so that the D-Fly remains high in the tube and not down by the metal BB.
Battery to D-Fly to Rest-of-Bike
This places the D-Fly in the seattube (Carbon) and not near the thicker and stronger seatpost or worse yet inside a metal drop bar..
Seems to work fine. I've even used it for a wireless firmware flash.

D-Fly did not come with a cable, get the shortest one you can,

Barry