First things first stay calm, Clean it by irrigating it with saline solution so you can see just how bad it really is to decide if you can treat it at home or it needs a vets attention. then apply either hibiclens, betadine or an antibiotic ointment. if it is very deep and you have access to a vet by all means you can , if it's not real deep you can usually treat at home at least 2 times a day.
Aww. Poor girl. Definitely disinfect it and no matter what...KEEP MAGGOTS AWAY. If you don’t think it would be stressful for her, you could take her inside in a cat carrier or something like that to monitor her. Chickens are very likely to peck at the wound, since chickens are attracted to the color red.
Hope this helps!