POST /as/token.oauth2 HTTP/1.1
  Host: authorization-server.example.com
  Authorization: Basic czZCaGRSa3F0Mzpoc3FFelFsVW9IQUU5cHg0RlNyNHlJ
  Content-Type: application/x-www-form-urlencoded

  grant_type=authorization_code
  &redirect_uri=https%3A%2F%2Fclient.example.org%2Fcb
  &code=10esc29BWC2qZB0acc9v8zAv9ltc2pko105tQauZ
  &resource=https%3A%2F%2Fcal.example.com%2F