Drinking Water and Worming Treatment

[KORKOR]

Hello,

I've recently purchased some Peperazine Solution for worming my two chickens who are about 20 weeks old.
I've been told by the pet store that I need to add 9.5mL of Peperazine per Litre of water.
The thing is, my chickens drink very little (which I believe is normal), so then how do I get them to drink enough water to make sure they get enough of the Peperazine solution for it to have an effect?

Drinking a few mL a day is not enough and the water will go stale and dirty by the time they drink enough of it for the Peperazine to have an affect.

Should I pour about 250mL of water and adjust the dosage accordingly to ensure it's all consumed?
(Sorry I'm still an amateur)

Thanks