odoo16/addons/auth_signup/views/webclient_templates.xml

<?xml version="1.0" encoding="utf-8"?>
<odoo>

    <template id="login_successful" inherit_id="web.login_successful">
        <xpath expr="//div[hasclass('oe_login_form')]/p" position="before">
            <p class="alert alert-success" t-if="account_created" role="status">
                Registration successful.
            </p>
            <!-- Remove parameter from URL, do not show "Account created" if page is refreshed -->
            <script defer="defer" type="text/javascript">
                window.history.replaceState({}, null, '/web/login_successful');
            </script>
        </xpath>
    </template>

</odoo>