You need to add your choice of emoji to the Emoji section, then click on the Save button at the end of the page.
This emoji will be displayed when you go to the user's profile page:
Now, if you want the emoji to be displayed in the content input box (the image you have shared), you can simply add the emoji to the beginning of the Singular and Plural name for the content type. In this case, you can simply remove the icon from the Emoji section and leave that section blank.
I hope I could have explained it clearly.