function getURLParam(strParamName){
  var strReturn = "";
  var strHref = window.location.href;
  if ( strHref.indexOf("?") > -1 ){
    var strQueryString = strHref.substr(strHref.indexOf("?")).toLowerCase();
    var aQueryString = strQueryString.split("&");
    for ( var iParam = 0; iParam < aQueryString.length; iParam++ ){
      if (
aQueryString[iParam].indexOf(strParamName.toLowerCase() + "=") > -1 ){
        var aParam = aQueryString[iParam].split("=");
        strReturn = aParam[1];
        break;
      }
    }
  }
  return unescape(strReturn);
}

function go_promo_code(url, default_promo_code){
  var url_promo_code = getURLParam('promotion_code')
  var new_url;
  if ( url_promo_code ) {
    new_url = url + "&promotion_code=" + url_promo_code;
  }
  else if (default_promo_code) {
    new_url = url + "&promotion_code=" + default_promo_code;
  }
  window.location = new_url;
}