To add a private note, you need to click on Private note in the reply window.
Note: This note will be visible only to the agents and not the customer.
You can also @ mention the agents in the private note.
Once you are finished typing the message, click on add note so that it will be published. Also, you can check the box Enter to send if you wish the note to be added when you click on the enter button.
Ex: This feature is helpful if you want to mention your team members so that they will have visibility over what is happening in the conversation or need assistance with any customer query in that chat.