Tracking visitor_sid with Shopify orders

example.html

      <input type="hidden" name="attributes[visitor_sid]" id="ctm_visitor_sid" value=""/>
      <script>
  function getCookie(name) {
    var nameEQ = name + "=";
    var ca = document.cookie.split(';');
    for(var i=0, len = ca.length;i < len;i++) {
      var c = ca[i];
      while (c.charAt(0)==' ') c = c.substring(1,c.length);
      if (c.indexOf(nameEQ) == 0) {
          return c.substring(nameEQ.length,c.length);
      }
    }
    return null;
  }
  document.getElementById('ctm_visitor_sid').value=getCookie('__ctmid');
        </script>