Hey @louise i​! Daniel from Customer Support Engineering here 🔧
Just to clarify, do you want a note to be left each time any reply is sent by a user? If so, you could use our conversation.user.replied webhook topic to detect whenever a user replies (there is an admin version, too). The conversation ID will be included in the returned payload. Then, follow up by creating a note in the conversation using our Reply to a conversation endpoint (specifying "note" as the type" and the ID of the conversation)!
Hope this helps 🙌