Hi,

Try the following code(which add a new menu item named "User Manual") for adding new menuitem on the top-right user menu

In .xml

<?xml version="1.0" encoding="UTF-8"?>
<templates xml:space="preserve">
    <t t-extend="UserMenu.Actions">
        <t t-jquery="a[data-menu='settings']" t-operation="after">
            <a role="menuitem" href="#" data-menu="manual" class="dropdown-item">User Manual</a>
        </t>
    </t>
</templates>

in.js

odoo.define('documentation.data', function (require) {
"use strict";


var page=require('web.UserMenu');
var ajax = require('web.ajax');
var rpc = require('web.rpc');
var usepage=page.include({
    _onMenuManual: function () {
        var base_url = window.location.origin;
        window.open(base_url + '/user_manual/', '_blank');
    },
    })

});

Regards