Dura Ace 12 speed connection issues

[Squirrel1966]

I have been trying to connect a giant TCR to my e-tube pro workshop software. I have been told that I need a junction box and, because all the cables are internal, i Should plug the box into the cable from the rear shifter, run another cable to the shifter from the box and then I can plug in my PCE02.
Can anyone please confirm this or explain the correct way to do it.