<link
  href="https://unpkg.com/grapesjs/dist/css/grapes.min.css"
  rel="stylesheet"
/>
<script src="https://unpkg.com/grapesjs"></script>
<script src="path/to/grapesjs-calendly.min.js"></script>

<div id="gjs"></div>

<script type="text/javascript">
  var editor = grapesjs.init({
    container : '#gjs',
    ...
    plugins: ['grapesjs-calendly']
  });
</script>