Well, as far as I know, there is no sure-fire way to get ducks to decide to make a home in any particular place.
That includes ducks raised domestically, even mallards.
Ducks are quite susceptible to predators, and those raised with people are at much greater risk for any number of reasons.
What I have seen, and would like to have everyone avoid, is ducks raised to expect food and shelter and protection, turned out with some food available but not much else, from their point of view. You probably know about hawks, eagles, snapping turtles, weasels, foxes, coyotes, raccoons, possums, maybe even bobcats.
A food plot with millet would attract waterfowl, based on the complaints I have seen from people trying to raise millet for themselves and their livestock!
Amaranth, wheat, oats, sorghum, are all seed-producing plants that are pretty in their own right, and have a reputation for drawing wild fowl. These foods are also much more nutritionally balanced than corn.