This guide will walk you through the steps to insert a comment (plain text between message bubbles) within your chat stories.
Navigate to the story where you want to insert a comment
Tap the pencil icon at the bottom right of the messages to enter edit mode.
The pencil button becomes an X, and editing options appear.

Tap the + icon at the position you wish to insert the message
Select Comment from the Insert menu that appears.
Type your desired comment line into the text field.
Press the Send button to the right of the text field to add the comment to the chat.
This article explains how to insert comments in your TextingStory creation by using the edit mode.