For those Googling for the answer like I was and finding this old thread, the polygon convert for floor/ceiling holes doesn't seem to be available in Home Designer Pro 2019 or 2020. Perhaps this only works or is available in Chief Architect's professional level products?
Instead, use either railings or invisible walls (room dividers) to create a room, then from the room properties change the room type to "Open Below" or under "Structure" uncheck the box for "Room under this floor". So for a hole in the ceiling, you'd do this from the floor above (or even attic level).
Official guide: https://www.homedesignersoftware.com/support/article/KB-00210/using-open-below-rooms-to-define-an-opening-to-a-lower-floor.html
