What type of harness?

If you are using a brook board you can use the brook fighting cable pack and plug the hitbox harness into the 5 pin on 20 pin.

If you are wanting braided sleeving then you will have to diy. This this the wiring guide .

I don't have an official hitbox so i'm not familiar with any special functionality, but if you treat it as building from scratch you could use the 20 pin harness and a hitbox cable.

the 20 pin by brook includes the tp/l3/r3 cable.

i hope this helps.