There are two different ways to add a “reaction” or “emoji” to a post or thread in the discussion forums.
One way is to react to the entire post: At the bottom right-hand corner of each post, there is a heart icon. Rollover the heart icon to reveal the different reactions that you can give to the post.
Include an emoji within your own post or response: Click on the emoji icon (happy face) in the text editor and it will pop out a menu of different emojis you can choose from. Click on the one you want in order to add it to the content of your post.