userscript.js

// ==UserScript==
// @name         Show Github Path
// @namespace    http://tampermonkey.net/
// @version      0.1
// @description  try to take over the world!
// @author       You
// @match        https://github.com/*
// @grant        none
// ==/UserScript==

(function() {
    'use strict';
    function onClick(e) {
        e.target.select()
        document.execCommand("copy");
    }
    const path = window.location.pathname.split('/').filter(e => e)
    const div = document.createElement('div')
    div.setAttribute('class', 'Header-item position-relative')
    div.innerHTML = `<div class="Header-item"><div class="js-site-search-form"><input type="text" class="form-control header-search-input" style="min-width: 40vw;" value="~/${window.location.host}/${path[0]}/${path[1]}" readonly /></div></div></div>`
    const input = div.querySelector('input')
    input.addEventListener('click', onClick, false)
    const priorSibling = document.querySelector('.Header-item--full')
    priorSibling.insertAdjacentElement('afterend', div)

})();