Prevention Worming - adding to water

I would like to worm my chickens for the first time. They are 9 months old. As far as I know they dont have worms, I do regularly examine their poo. As they free range alot I thought that the liquid that you add to water would be the best bet to make sure they have enough of it. Is there a liquid one where you dont have to withdraw the eggs for a while.
Trying checking on the web but the old computer is a bit slow and I know there are plenty of experts on here that can recommend products they have used already.
Thanks

how do you manage the dosage etc. must admit this appeals to me more. books do often seem to be a bit rigid