You can sprinkle it over their food.
If you wish to treat a whole flock for sour crop, or as a preventative, I suggest using copper sulfate to treat their water. It would be much more cost effective and easier to administer and just as effective.
If it comes in a capsule form you have the option of popping the pill directly into her beak or opening the capsule and dividing it into two doses. What I do to make getting the loose powder into the chicken easier is to mix each dose into a "pill" of coconut oil, chill until solid, then pop the...
Nystatin is an anti-yeast med, but it doesn't treat all strains of Candida. If that's all you can get, then use it. You could get lucky and it will knock out the type of yeast plaguing your chickens.
To clarify, Epsom salt solution is a drench or a flush and does not kill yeast. It stimulates gentle diarrhea to expel yeasty material. I recommended it as an alternative to people living in countries where anti-yeast meds are not readily obtainable, especially in Europe where miconazole is...
Here's a pigeon supply web site that sells Medistatin at a good price. http://uspigeons.mercasystems.com/index.php/medpet-pigeons-products-medistatin-powder-treatment-of-candida.html