You can use record.create: to create a comment on any record.

Create a formatted comment as Cerb on a ticket record

• automation
• policy

• 
  start:
    record.create/comment:
      output: new_comment
      inputs:
        record_type: comment
        fields:
          author__context: app
          author_id@int: 0
          target__context: ticket
          target_id@int: 123
          is_markdown@int: 1
          comment@text:
            This is a **comment** from an automation.
  
  

  Field
  author__context:	record type of author (app, role, group, worker)
  target__context:	record type to comment on (ticket, message, task, etc.)

• 
  commands:
    record.create:
      deny/type@bool: {{inputs.record_type is not record type ('comment')}}
      allow@bool: yes