Ducks don't seem to have the ability to go in a certain spot. The closest to that is when a female duck goes broody, sits on eggs on a nest, and then she won't defecate on the eggs, she holds it till she can get off the nest. But that is the exception. Generally, they just go where they stand or sit.
Amykins recently made a diaper setup for Wobbles, and Nettie has been making diaper holders for years. There are others on the forum who have done the same.
Im sorry but ducks and geese actually don't have rectum muscles so when they got to go it just comes out, they can't control it, its a mechanism to help the sperm travel to the egg faster as well, and they tend to poop every 12-20 minutes so they would be at the little box all day, diapers are the way to go but they don't really make baby sizes, more as to adult duck or juvenile sizes
Yes, raising ducklings in the house is just a poopy business altogether. Many people here have house ducks and use duck diapers though so they can move freely inside without making a mess. It's just not possible to toilet-train a duck, unfortunately.