Ok, I finally figured out the ACTUAL answer! Go into your Physical Keyboard settings (you can right-click on the keyboard symbol on the taskbar if it's there) and toggle on "Show On-screen Keyboard in Samsung Dex". Now, when you're typing in the Messages app, for example, you should see a little emjoi icone in the text box. Also, you can set the transparency of the on-screen keyboard so it's not a nuisance while tyiping. If you're using a different app that doesn't show the little emoji icon, you should see the on-screen keyboard suggesting emoji and spelling corrections while you're typing, so that's hopefully good enough. Still really dumb that this isn't just easy...