Steam store auto add to cart game

// ==UserScript==
// @name         auto click
// @namespace    http://tampermonkey.net/
// @version      1.9
// @description  auto click pay button
// @match        *://*/*
// @author       peter
// @grant        none
// ==/UserScript==

//setTimeout(beginclick,30000);
function simulateClick(obj) {
  var evt = document.createEvent("MouseEvents");
  evt.initMouseEvent("click", true, true, window,
    0, 0, 0, 0, 0, false, false, false, false, 0, null);
  var canceled = !obj.dispatchEvent(evt);

}
var whats = document.getElementsByName('subid')[0].value;
var name ='a[id="btn_add_to_cart_'+whats+'"]';
var what = document.querySelector(name);
simulateClick(what);

<div class="cart_item_list">
all html code between this section
</div>