Within Lex, how do you create a new line in the prompt, to add spacing in the message reply? I want to line break a prompt?

It seems to depend on the Channel being used, here are some of the things that you should try: \r\n \n %0a

To create a new line in an Amazon Lex prompt, you should use SSML (Speech Synthesis Markup Language) tags in your response text. When configuring the response for an intent in Lex, you can include your text within SSML tags and use break tags to create pauses or line breaks.

For example, you can format your prompt like this:

<speak>
Does this work
<break time="500ms"/>
Yes or No?
</speak>

The <break> tag creates a pause that effectively serves as a line break in the conversation flow. You can adjust the time value to control the length of the pause.

You can add this SSML markup directly in the response section when configuring your intent in the Lex console. This approach allows you to format your responses with appropriate spacing and line breaks for a better user experience.
Sources
Using ssml in lex | AWS re:Post