I have very little experience and please listen to others if anyone offers, but I would provide some sort of privacy for the hen.  Like a bucket or box (crate) turned backwards in a corner, just far enough away from the wall for her to get into and some bedding.  If you lock up your ducks at night they probable do not know where to go for a nest.  Then I would put the egg they layed outside in the nest marking by date layed or someway to identify it.  They may get the idea with other eggs in the nest.  Good luck!  calls eggs are a b*^$h to hatch.