If you're using Amethyst, what you can do is go to my reacts and add them to your app. Then when you long press the heart button, you can add the custom emojis so you can react with them.

On clients like Nostrudel, you can even put them into your notes, but only a few clients will actually render them. Amethyst will show them, but you can't write a post with them. Only do reactions.