if you're on mobile, you can just do it like a normal emoji, I think? You can always make a private only-to-you post in one of your journals and then comment with a regular text emoji (like from your phone emojis) and see if it works
😀😃😄😁😆😅🤣😂🙃 Normal phone emojis work. I'll be honest, if I'm on my laptop, I often either respond with my phone or copy and paste someone else's emoji if I want one while on desktop.