Example 

---
rules:
  - name: Single Line
    on:
      path: /content/1
      methods:
        - get
    answer.content: Hello World

  - name: Multi-Line
    on:
      path: /content/2
      methods:
        - post
    answer.content: |
        Line 1
        Line 2
        {{ .Input.Form.Text }}

To see it in action, execute requests as follows:

$ curl localhost:3000/content/1
Hello World

$ curl localhost:3000/content/2 -F Text="Hello World"
Line 1
Line 2
Hello World

Read more about how to use placeholders.