Add CORS header to current Irule.

i need to add a CORS header to this Irule. Can someone tell me where it would go?

The CORS header i need added is Access-Control-Allow-Origin: *

Here is our current Irule.

when HTTP_REQUEST {

  set json "{ \"message\": {\"title\": \"System Maintenance\", \"body\": \"System is undergoing maintenance.\", \"summary\": \"Please try again later.\" }}"

HTTP::respond 418 content $json "Content-Type" "application/json"

TCP::close

}