mail-addons/mail_private/template.xml

<?xml version="1.0" encoding="utf-8"?>
<odoo>
        <template
                id="assets_backend"
                name="mail_private_assets_backend"
                inherit_id="web.assets_backend">
            <xpath expr="." position="inside">
                <script
                        type="text/javascript"
                        src="/mail_private/static/src/js/mail_private.js"></script>
                <script
                        type="text/javascript"
                        src="/mail_private/static/src/js/test_private.js"></script>
            </xpath>
        </template>
</odoo>