B complex is inactivated by light- typically when I've seen it sold its in an amber vial. So, if the powder is designed to be given in water or feed, it probably doesn't do much unless its protected from light. If I thought my chicks needed it supplemented, I would give it directly to know that they receive the administered dose. Is there a reason you're supplementing it?
B complex is water soluble and very safe to give, but I don't have an actual chicken dose. Probably doesn't need quite as much as an adult person; B complex is not stored in the body, so if overdosed the chicken should just excrete the excess.