Peat moss on the bottom layer I believe and then sphagnum moss- dried. Don't plug the hole. That's what I read. The sphagnum moss is what eventually turns into peat moss as it decomposes but you can grow live sphagnum on the top layer.

How deep is this sink?? I would check out a local pet shop and snag one of those foam filters for fish tanks and use it to plug the drain hole in the sink as it will let the water out but not the substrate.
The substrate should be 50 50 river sand and peat moss.
You can plant live sphagnum into it. It will look nice but be sure it gets 2 to 3 hours of direct sun, preferably in early morning or late afternoon unless you are up in the northern USA then mid day is fine.
Do not overplant!! Probably just 3-5 species of carnivorous plants and nothing else apart from live sphagnum and you will have a nice bog.