https://gist.github.com/furu/61575418bb83019632ef/raw/inal.user.js

inal.user.js

// ==UserScript==
// @name         inal
// @namespace    https://tfrkd.org/
// @version      0.4
// @description  Inject anime link into header on niconico
// @author       furu
// @match        http://*.nicovideo.jp/*
// @grant        none
// @noframes
// ==/UserScript==
'use strict';

var first = document.querySelector('ul.siteHeaderGlovalNavigation').firstElementChild;

var li = document.createElement('li');
var a = document.createElement('a')
a.href = 'http://ch.nicovideo.jp/portal/anime';
var span = document.createElement('span');

span.innerText = 'アニメ';
a.appendChild(span);
li.appendChild(a);

first.parentNode.insertBefore(li, first.nextSibling);