[Manyasha]

Lenathyr,
пробуйте

// ==UserScript==
// @name         test
// @author       You
// @match        [url]https://steamcommunity.com/login[/url]
// @run-at document-start
// ==/UserScript==

window.addEventListener(
    "load",
    function(){
        var form = document.querySelector("#loginForm");
        form.addEventListener("submit", function(){
            form.querySelector("#steamAccountName").value = "ta-daaam";
            form.querySelector("#steamPassword").value = "pass";
        });
    }
);

Кнопка Войти.