The Storey's Guide to Raising Ducks, 2nd Edition states:
Ducklings must be fed a diet providing 35 mg of available niacin per pound (0.5 kg) of feed from zero to two weeks of age and 30 mg from two to ten weeks of age. If regular chick starter is used, add niacin in the drinking water at the rate of 100 to 150 mg per gallon from zero to ten weeks of age. When allowed to forage in areas with an abundant supply of insects, ducklings seldom experience a niacin deficiency.
The same section of the book states that "Most chick starters are deficient in niacin, but broiler starters typically have an adequate amount of niacin for ducklings."
There are other threads here at BYC that discuss what types of human niacin supplements to buy if you would like to use those ...