I don't know about making them go broody, but I always keep wooden eggs in all my nesting boxes and remove the real ones. I feel like I am killing several birds with one stone. First, it teaches them where to lay. Then they think it is an egg and start brooding sometimes. There are a few other reasons I do that, but they are irrevelant to this topic. There is no guarantee. No amount of sweet talking will work. I just keep at least a handful of chickens that are known for brooding along with my layers, so I also won't have to incubate, because I am terrible at it. Give it a little while, and count down the days, and if no luck, start the incubation. I don't blame you for wanting a silkie to do the job for you......duck eggs are harder to hatch out than chicken's.