Can you leave sticking out of the slot?

That’s the point of the post-it note. So they see it.

I’ve done it before at an apartment, though admittedly it was many many moons ago.

If you don’t want to take it to a mailbox, you can put it back in your own mailbox with the “return to sender - no longer at this address” on it. The mail carrier can pick it up when they deliver your mail next. If they seem to miss it, you can try putting a bright post-it note on it to make it stand out.