Tampermonkey/GreaseMonkey script

// ==UserScript==
// @name CheatInterface
// @namespace Cookie
// @include orteil.dashnet.org/cookieclicker/
// @version 1
// @grant none
// ==/UserScript==
var oldOnload = window.onload;
window.onload = function () {
oldOnload();
var script = document.createElement('script');
script.setAttribute('src', '<source link>');
document.body.appendChild(script);
}