How do you fill dango (Japanese boiled treats on a stick)?

I would like to make some pork belly-filled dango for a meal I’m making, but I can’t find any recipes online of a filled version, only unfilled. It seems simple enough, but I’m not sure whether to fill before or after boiling the dango. If I filled before, would the dango not float? Is it important they do float?

Thanks for your help!