I think I know what you mean Math. It's an integral overflow, much like a bathroom basin waste. It needs a "slotted" waste. Sometimes the right sized waste for imported sinks isn't slotted, which is why the plumber drilled the hole.
I think Naz is thinking more of a stainless steel sink, which would seal with a rubber washer. If I'm right about yours, then silicone on each side of the washer is often used. Just hand-tighten the backnut though, or you'll squish out all the silicone.
Everyone has different ways of doing this.............. other methods are available :o))))