This seems a bit vague to me. You may want to talk to the vet for clarification. Rooster booster is a brand. They could be taking about rooster booster poultry cell, rooster booster vitamins and electrolytes and I can go on with this list.
So Naicin is a B vitamin. The B12 you mentioned long ago in the thread just had B12. I would agree and go for a B complex, which contains a variety of b vitamins including naicin. Did tell your vet you have been giving your duck naicin?
If you have a pill, you will just want to stick it down it's throat. You can post a picture of the ingredients and we can double check the dosage.
This is usually what most of us use from TSC, and it is 1ml in a syringe right into it's throat.
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Also if you are confused at all what the vet is saying, contact them and ask for my specifics and hands on examples. It is not your fault if you don't understand, they should be explaining things better.