Do you want to reply directly to the notification zammad sends to the agent and expect this reply to reach the customer?
When editing templates for e.g. triggers, you can integrate placeholders just like you would to in a normal ticket:
You type “::” and a selection will appear. Continiung typing will bring you nearer to what you search for.
As for giving the whole ticket content to the customer: This is currently not possible, you can also see: