Depends on if you are trying to use an emoji from the Internet or one from the vast selection of emoji smilies here on EO. If it's one from the Internet, you'll have to post it like you would a picture, by clicking the picture icon on the editor toolbar and then pasting the URL into the dialog box. The picture will then be placed where the cursor in in the message window.
If it's one of the smilies, click the smilie icon on the editor toolbar and then below the text box select the smilie you want to use. It will likewise be placed where the cursor is in the message box.