How do I automatically insert some text in the comment textbox on submit?

We can submit a comment without text as this function allows the user to submit a comment without any text in the textarea. Now all you need to do is hide the comment textarea with some display:none; css.

    if ($comment_data['comment_content'] == '%dummy-text%') {
        $comment_data['comment_content'] = ''; // replace dummy text.
    }
    return $comment_data;
}
add_filter('preprocess_comment', 'rei_preprocess_comment');
function rei_wp_footer() {
    ?>
    <script>
    jQuery(function($){
        var comment = $('textarea#comment');
        comment.removeAttr('required'); // remove required attribute of textarea.
        $('#commentform').on('submit',function(){
            if (comment.val() == '') {
                comment.css('text-indent','-999px').val('%dummy-text%'); // change to dummy text.
            }
        });
    });
    </script>
    <?php
}
add_action( 'wp_footer', 'rei_wp_footer' );