That is cattle dewormer and isn't labeled for use in chickens. I personally would not put it on my birds. It is a poison that is very hard on the system. I would maybe never use it on a hen that had been sitting for long already. To me they are in a compromised state and I won't subject them to that. Especially since it might cause diarrhea or something and broody's try to leave the nest very seldom.Would Ivermectin as a spot on be better?
I use the permnethrin spray on my broody's, but they don't usually get eggs (bet yes even if I am tucking chicks right away or they already have them). I would just use it when booting her off the nest for a minute and let her return as usual. If it is damp it will dry and cause no harm in my PERSONAL opinion. Otherwise spray her after hatch... but I would do it now, and not fret.