openapi : 3.0.0
info:
  title: A simple webhook subscription
  version: 1.0.0
paths: 
  '/subscribe':
    post:
      requestBody:
        content:
          application/json: 
            schema:
              type: object
              properties:
                url: 
                  type: string
                  format: uri 
      responses:
        '201':
          description: Created subscription to webhook
      callbacks:
        mainHook:
          '$request.body#/url':
            post:
              responses:
                '200':
                  description: webhook successfully processed