In order to do this, the fixture should be hung independently from structure prior to the ceiling being installed. It is also recommend the TG trim so the fixture can float in the ceiling opening. The contractor will need to set the bottom of the trim level with the finished ceiling height, and the edges of the fixture opening will need to be finished. The bottom of the TG trim will create a small reveal around the edge of the fixture, so part of the horizontal edge of the fixture cutout will be visible.
Submittal drawing and RFD is required because this would be using a TG trim with a GB or FL mounting method.