SteamSimilarGameButton.js

// ==UserScript==
// @name         Steam Similar Game
// @namespace    undefined
// @version      0.1
// @description  Add a Similar Game Button
// @author       Hippo Seven
// @match        http*://store.steampowered.com/app/*
// @grant        none
// ==/UserScript==

var id = getId();

var button = document.createElement('div');
button.setAttribute("class", "apphub_OtherSiteInfo" );
button.innerHTML = '<a class="btnv6_blue_hoverfade btn_medium" href="https://store.steampowered.com/recommended/morelike/app/' + id + '/"><span>类似产品</span></a>';
button.style.marginRight = "6px";

appendChild(document.getElementsByClassName("apphub_HeaderStandardTop")[0], button, 1);

function getId() {
    return document.getElementsByClassName("glance_tags popular_tags")[0].getAttribute('data-appid');
}

function appendChild(parent, child, index) {
    parent.insertBefore(child, parent.children[index]);
}