You can only react with emojis to messages in internal chats, not in external chats with visitors.

To react to a message, you hover the mouse cursor over the message. The field becomes grey and in the field's upper corner you see a smiley with the text Add reaction:

You click the smiley and the view below appears:

You click the emoji that you want to to react with. In this example, Matilda chooses the smiley with two hearty eyes  . This reaction then becomes visible for everybody in the room:

If someone else reacts to the message witht the same emoji, it says 2 instead of 1 next to the emoji reaction as two people then have reacted to the message with the same emoji. Though, in this exmaple, Emma reacts to the same message with a thumbs up. This is how it looks like when two people have reacted to the same message with different emojis:

You can hover the mouse cursor over the reaction field to see who the person behind the reaction is.