You don't need a seal on the underside, it should seal on the top - inside the sink.
You do need to seal the underside and the top. The drain tube has slotted holes in it where the overflow drains back. If you don't get a seal on the underside water will leak through those slots and out around the cone washer. I just fought a drain leak like that when I installed a new faucet and drain. The new cone washer was too big to go up through the hole in the sink. I finally used the old cone washer and plumbers putty.

--
For full context, visit https://www.homeownershub.com/mainte...ue-415394-.htm