WordPress AJAX load post metadata in modal

you can use ajax url creating methode of wordpress.

add_action("wp_ajax_clientLogoPopup", "clientLogoPopup");
add_action("wp_ajax_nopriv_clientLogoPopup", "clientLogoPopup");

    function clientLogoPopup()
    {
        $clientID = intval($_REQUEST['client_id']);
        if($clientID > 0){
            echo get_post_meta($clientID , 'client_logo', true);
            // or you can also show other meta fields here.
        }
        die();
    }

write above script in functions.php of your theme

now call the url as per your requirement like this,

http://www.your-website.com/wp-admin/admin-ajax.php?action=clientLogoPopup&client_id=