lazy-zendesk.js

(function () {
	
    function loadZendeskChat(callback) {
      var zdscript = document.createElement('script');
      zdscript.setAttribute('id','ze-snippet');
      zdscript.src = 'https://static.zdassets.com/ekr/snippet.js?key=XXX-XXX-XXX-XXX';
      (document.getElementsByTagName('body')[0]).appendChild(zdscript);

      window.zdonload = setInterval(function(){
        if(typeof zE !== "undefined" && typeof zE.activate !== "undefined") {
          clearInterval(window.zdonload);
          callback();
        }
      },50,null)
    };
    window.loadAndOpenZendeskChat = function() {
      var button = document.getElementById('zdbutton');
      localStorage.setItem('ff_zd_hasOpened',true);
      button.innerHTML = 'Loading...';
      loadZendeskChat(function(){
        window.setTimeout(function(){
          zE.activate();
          button.parentNode.removeChild(button);
        },1000);
      });
    }
    if(localStorage.getItem('ff_zd_hasOpened')) {
      loadZendeskChat(function(){});
    } else {
      document.getElementsByTagName('body')[0].insertAdjacentHTML('beforeend','<button id="zdbutton" aria-label="Launch Help Chat Window" onClick="window.loadAndOpenZendeskChat();">Help</button>');
    }
}());

Just a little update, rather than introducing a race condition with the setTimeout, you can wait for zE to be ready:

zE(() => {
    zE.activate();
    button.parentNode.removeChild(button);
});