The $params array using Unique Arguments

WebAPIunique

$params = array(
            'api_user'  => $user,
            'api_key'   => $pass,
            'to'        => $adresa,
            'subject'   => $subiect,
            'text'      => $mesaj,
            'html'      => $html,
            'x-smtpapi' => '{"unique_args": {"campaign_id":"121"}}',  
            'from'      => $from
        );