How do I lock the trunk on a 2018 Hyundai Kona?

Read your owners manual but I am pretty sure the trunk button is a release. To lock it use the lock button.

I have the same issue. When I press the lock button on my key, the doors lock and I can't open them by pulling the handles, but I *can* open the boot (trunk) just by pressing the release on the boot with my fingers.

I'll just bet you still have your key fob with you when you're doing this. Try locking the doors using the lock button on the key fob, then take the fob 30-40 feet away and try opening the doors. Don't forget to try pressing the little button on the door handle first, and nothing should happen. Also, I notice that when I have my fob with me, I have to press the tailgate button twice in order for it to open. I think the first press unlocks it, and then the second press opens it. Now, I *have* noticed a couple of little bugs in the programming having to do with door locks seeming to jam if the engine is running and you accidentally don't close the tailgate all the way. After trying a few magic incantations with no success, I found the way to cure this is to stop the engine, wait 30 seconds, and start it again, so the system will reset itself. Same goes for any other situation where the locks don't do what you want -- start, stop, then restart, just like you would with any other computer. Just think of your Kona as a big computer, and you'll find any frustrations will magically go away.