You've gotten lots of good answers, but I can't resist chiming in.
Please forgive my mention of something obvious, but I want to confirm that you are not planning to drill a hole large enough to fit the plug of the cord through. You'll keep the hole tiny, just barely large enough to force zip cord or regulation wire through, and then mount an outlet box inside, right? That's what I did. An outlet box secured to a stud is a lot better than a dangling cord.
Also, someone recommended expanding foam. I avoid that stuff, because after a year or two it will dry out, crumble, and eventually turn to dust.
Another consideration is friction wear. Will your wire blow in the wind or be rubbed by chickens? If it rubs back and forth against a sharp bit of wood in the hole, the insulation may wear away and eventually cause a short that may set your coop on fire. I recommend that you staple the wire down, inside and out, to prevent all movement.
Hope this helps!
Tim