You can do either. Panacur has a very high safety margin.
I actually put it in their food - usually in the morning when they are hungry and will eat it all in one go.
As you will be aware poultry are monogastric, like dogs, unlike sheep and cattle. It is recommended to give panacur to dogs over 5days, instead of just one as in ruminants. So I looked up the dose rate for dogs but can't remember where!
Anyway for monogastrics you give about the same daily rate as you would for say a sheep, but you give it for 5 days. So effectively they get 5 x the dose in total. (Don't worry - you won't overdose them. It's because feed passes differently through a dog or hen.)
So, as I say, you can put it in either feed or water which is easier than handling each hen.