web/web_hideleftmenu/static/src/xml/lib.xml

<?xml version="1.0" encoding="UTF-8"?>
<template>

    <t t-extend="UserMenu">
        <t t-name="web_hideleftmenu.hidemenu" t-jquery="ul.dropdown-menu li:last" t-operation="after">
            <li class="oe_hidemenu">
                <a href="#">Hide/Show Menu</a>
            </li>
        </t>
    </t>
</template>