<script>
    document.addEventListener('readystatechange', function(state) {
        if(state.target.readyState === 'interactive') {
            let emailField = document.querySelector('input[name="Mail"]');
            if(typeof emailField !== 'undefined' && emailField !== null) {
                emailField.value = 'foo@bar.de';
                emailField.disabled = 'disabled';
            }
        }
    });
</script>