warmshowers-message-link.js

// ==UserScript==
// @name         Warmshowers.org - Replace "Donate" with "Messages" Button
// @namespace    http://tampermonkey.net/
// @version      0.1
// @description  try to take over the world!
// @author       You
// @match        https://www.warmshowers.org/*
// @grant        none
// ==/UserScript==

(function() {
    'use strict';

    document.querySelector('.is-leaf.last').outerHTML = '<li class="menu__item is-leaf leaf"><a href="https://www.warmshowers.org/user/10304/messages" title="" class="menu__link">Messages</a></li>';
})();