add_action( 'comment_form_logged_in_after', 'review_field_fields' );
add_action( 'comment_form_after_fields', 'review_field_fields' );
function review_field_fields()
{
    ?>
    <p class="comment-form-url">
        <label for="url"><?php _e( 'URL' ); ?></label>
        <input type="text" name="url" id="url" />
    </p>
    <?php
}
add_action( 'comment_post', 'review_field_insert_comment', 10, 1 );
function review_field_insert_comment( $comment_id )
{
    if( isset( $_POST['url'] ) )
        update_comment_meta( $comment_id, 'comment_author_url', esc_attr( $_POST['url'] ) );
}