Desktop:
1. Open your conversation
2. Hover over to the message you'd like to reply or comment on.
3. Click on the 
comments/reply icon next to the message.
4. Type your message in the Write a comment... field.
5. Press Enter/Return key to send your message.
Mobile:
1. Open your conversation.
2. Tap on the message you'd like to reply or comment on.
3. Type in your message in the Write a reply... field.
4. Tap on the 
send message icon to send your message.