WordPress custom form action url

simple with jQuery:

//first make sure you have jQuery on that page
add_action('wp_enqueue_scripts','make_sure_i_have_jquery');
function make_sure_i_have_jquery(){
    if (!is_admin())
        wp_enqueue_script( 'jquery' );
}
//then just change the url to you own
    add_action('wp_footer','change_comment_form');
    function make_sure_i_have_jquery(){
        if (!is_admin() && (is_page() || is_single()))
            echo '<script> $("#commentform").attr("action", "http://yourUrl.com"); </script>';
    }

just paste this in your theme’s functions.php or the plugin your are develop and change http://yourUrl.com to the url you want