Load script with callback

loadScript.js

function loadScript(id, url, callback) {
  const script = document.createElement('script');
  script.type = 'text/javascript';
  script.id = id;
  script.src = url;
 
  // Several events for cross browser compatibility.
  script.onreadystatechange = callback;
  script.onload = callback;

  // Appending the element will trigger the script load
  document.head.appendChild(script);
}

// source https://develop.zendesk.com/hc/en-us/community/posts/360001652887-Web-Widget-API-Hide-Launcher-Button