Contact Form 7 watermark for select menu

Put this code in your functions.php file:

function my_wpcf7_form_elements($html) {
$text = 'Please select...';
$html = str_replace('---', '' . $text . '', $html);
return $html;
}
add_filter('wpcf7_form_elements', 'my_wpcf7_form_elements');