This is copied from the article linked above:
"
How to administer
There are several ways to administer the vitamin B complex.
For novice duck keepers we recommend giving the complex over treats, this is a fairly safe approach to the administration of the complex as not much can go wrong. Ducks seem to enjoy peas, mealworms, watermelon, and lettuce the most. Simply, mix the complex into the treats. If the ducks don't take kindly to the taste, you can split the dosage up several times throughout the day so the taste isn't as potent in one sitting. For a list of safe treats for ducklings please read the bottom of this post.
If you're unable to get the ducklings to eat the treats, or they're in such bad shape you want a sure/fast way of administration, you may consider administering the complex orally.
Orally (PO), simply involves giving the medication directly into the duck's mouth. The tinier the syringe, the easier it is to administer with. I fill a 1ml syringe with the complex, grab the duck, I gently pinch the external corners of its mouth to get it open, and stick the syringe down the bird's right side, and slowly squeeze the plunger.
NOTE, it is very easy to aspirate a duck when giving medication by mouth, especially ones that are on the younger side. So, please do your research and read this article thoroughly before administering it.
Administering Oral Fluids
Per "
Metzer Farms", you can administer niacin capsules, via water, given at a ratio of 500mg per four gallons of drinking water. As with adding any supplement to their water, another clean water dish should be provided, in case the ducks refuse to drink the water.
The Complex can also be given (SQ) Subcutaneously, and (IM) Intramuscular. I'm not going to go into depth on how to administer these two ways since it can be harder to perform then the methods mentioned above, and more "scary" for new owners. If you want to learn more about these two methods of administration, please refer to the bottom of this post."