It will not hurt the fish regardless of how you add it. In fact, in my native tank, my LMB is very curious, and often mistakes me adding the first capful of prime for me throwing in food, and will often get a huge face full of prime.
I add it directly to the tank. Make sure to dose for the total tank volume, as the directions on the bottle indicate. I do 66% changes weekly with no ill effects, though I "overdose" slightly.