just paste this code in “Script in the body”

<script type="text/javascript">
document.addEventListener('DOMContentLoaded', function() {
  var script = document.createElement('script');
  script.type = 'text/javascript';
  script.src = '//cdn.callrail.com/companies/*******/4b1f35b918a351fe5d51/12/swap.js';
  document.body.appendChild(script);
});

</script>