web/web_widget_url_advanced/templates/assets.xml

<?xml version="1.0" encoding="utf-8"?>
<!--
Copyright 2018 Simone Orsi - Camptocamp SA
License LGPLv3.0 or later (https://www.gnu.org/licenses/lgpl-3.0.en.html).
-->
<odoo>
    <template id="assets_backend" name="web_widget_url_advanced assets" inherit_id="web.assets_backend">
        <xpath expr="." position="inside">
            <script type="text/javascript" src="/web_widget_url_advanced/static/src/js/url_widget.js" />
        </xpath>
    </template>
</odoo>