In my opinion. Nothing.
They are use to navigating around their enemies in the wild. If they were to smell coyote urine for example around your coop they will approach cautiously but once they don't see a coyote around in for a chicken dinner they go.
The best urine to deter a fox is your own. If your urine storage and distribution equipment is of the proper type, sprinkle your own urine liberally on and around the still warm foxes' body. I have always first had to applied my 12 gauge scatter gun to the foxes' chicken killing hide to convince him to stand still long enough for me to do this deterrent treatment. But I have never had even one fox so deterred kill another one of my chickens. It's 100% effective as well as being very emotionally satisfying.
In fact fox urine is sold by trapping supply houses to ATTRACT foxes and every other predator TO the trap.