There are available recipes for organic foods and also medicated & unmediated versions available at the typical feed stores in rural areas. Many times, you have found this helpless hungry creature and you aren’t necessarily trying to keep it as a pet but do feel the need to provide it with some nourishment, kudos to you but be careful. Animals that grow accustomed to being given food rather than having to forage for it are learning dependency and this can cause future issues in their lives.
One person reports their ducks favorite foods are of grass and dirt in the yard, canned corn, canned carrots, and of course bugs like grasshoppers, wasps, crickets, whatever it is that flies into their pool or water and can't escape. Their favorite way to eat is to sieve the water through their beak and get the tiny pieces. So, if offering them grass or hay they wouldn't eat it from your hand but if you just dump a bunch in the pool or water bowl, they will eventually eat it. While this person sounds like they really loved their pet duck, it’s never recommended to feed any canned or processed foods to animals that are wild. Pets are different and as long as the vet gives them a clean bill of health, it should be fine.