Using |indent

In this example we quote a prior email message by adding the standard > character to the start of each line.

• automation
• output

• 
  start:
    set:
      original_message@text:
        This is quoted text
        From a prior conversation
        and we want to quote it
    return:
      new_message@text:
        {{original_message|indent('> ')}}
        
        ... and this is our reply.
  
  

• 
  __return:
    new_message: |-
      > This is quoted text
      > From a prior conversation
      > and we want to quote it
  
      ... and this is our reply.