We recommend for novice duck keepers to give them the complex over treats, to reduce the risk of aspiration if given orally with a syringe. Duck seems to enjoy mealworms, peas, lettuce, tomatoes the most, it can also be mixed into a wet soupy feed. The reason we recommend not giving it in water...
The capsules are "flush-free" and thus won't work since they don't stay in the system long enough to be properly absorbed, you need plain niacin pills.