I had an eggbound chicken (not duck) last night, for the first time ever. We are also duck owners, but they are not old enough to lay.
My husband and I ran a warm bath, and put epson salt and lavender in the water to help it relax. However, with a duck, I'm not sure I'd do lavender unless you can keep it from drinking the water. My husband felt her abdomen gentley, and felt that there was indeed an egg there. He put on a rubber glove, lubricated the glove in vaseline, and inserted his finger into her vent. He did not feel anything which told us that we were probably in the early stages of her being eggbound. I ran to the store and grabbed some calcium gel tabets. When I came home, we cut them open and syringed the liquid to her. It was 1,200mg but we were desperate and not all of it got in her. Then we inserted some preperation h in her vent, to reduce the swelling so there was room for the egg to come out. Then, we put her in a dark room by herself until she passed the egg.
The dark tells the "egg maker" to stop making new eggs. So, if you can, leave your duck in a dog crate or something in a dark room instead of letting her stay in the light.
I wouldn't feed her too much food because all of that food and water has to come out the same hole that the egg comes out. Is she still able to poop? It seems like that's when this gets really life threatening.