To add a new line to the default text area in messageML, you can use the HTML entity .
For example, the code in Figure 1 will be displayed as shown in Figure 2:
Figure 1 MessageML code
Figure 2 MessageML display
Note: When the form is submitted, the new lines will be sent to the bot as a "\n".
Refer to the Knowledge Article found here for further information on the text area element.