Macil

// This code is assumed to be part of one of the extension's content scripts

function addIframe() {
  const iframe = document.createElement('iframe');
  iframe.addEventListener('load', () => {
    iframe.contentWindow.postMessage({
      eventName: 'init',
      iframeSrc: 'https://xkcd.com/' // whatever url you want. Probably will be a URL to your own domain in practice
    }, '*');
  }, false);