You didn't say specifically, but I'm assuming you mean a wire mat under the pen, not under the coop (hen house), is that right?
I would suggest installing the wire as an apron around the perimeter rather than below the run. That means you would either continue the wire down the side of the pen, bend it at ground level and continue out 18"-24" OR you would attach an 18"-24" strip of wire fencing to the bottom of the pen and lay it on the ground around the pen. You can cover it with dirt or grass if you like, but this will deter animals that will try to dig down and into the pen. There must not be a gap between the side of the pen and the beginning of the apron.
Also.... make sure the bottom 2 - 3 feet of the pen is covered with half inch hardware cloth not chicken wire. Anything with a hole bigger than a half inch and the racoons can reach inside and grab your chickens and rip them to shreds. The wire needs to go up high enough that it's taller than the chicken. Hungry racoons are incredibly tenacious and clever.
Enjoy your new chickens, they're a total hoot! And welcome to BYC!