Do I have to add some kind of seal o cement around the tube orifice in the base of the sink?

Pictures: www.google.com/search?tbm=isch&q=installing+a+sink

When I did that job I used plastic pipes, so I eventually had to glue all the joints to get rid of leaks. But there are some rubber washers here and there and you have to include those, glue or no glue. It is possible to use "Silicone Rubber Tub And Tile Sealer" to seal a joint if you don't have a rubber washer for it, but you have to be plenty graceful to do that. If you are the type to get glue on your fingers, get somebody else to do it.