web/web_widget_heatmap/views/web_widget_heatmap_template.xml

<?xml version="1.0"?>
<!-- Copyright 2019 Dinar Gabbasov <https://it-projects.info/team/GabbasovDinar>
     License AGPL-3.0 or later (https://www.gnu.org/licenses/agpl). -->
<odoo>

    <template id="assets_backend" inherit_id="web.assets_backend">
        <xpath expr="." position="inside">
            <script type="text/javascript" src="/web_widget_heatmap/static/src/js/heatmap_widget.js"></script>
            <link rel="stylesheet" type="text/css" href="/web_widget_heatmap/static/src/css/heatmap_widget.css"/>
        </xpath>
    </template>

</odoo>