void LoginWindow::on_pushButton_clicked(){
    // Setup the webservice url
    QUrl serviceUrl = QUrl("http://urlrestservice.com");

    QByteArray postData;
    postData.append("request=userdata&");
    postData.append("user=myusername&");
    postData.append("password=mypasswod");

    // Call the webservice
    QNetworkAccessManager *manager = new QNetworkAccessManager(this);

    //QNetworkRequest::setHeader();
    QNetworkRequest request(serviceUrl);
    request.setHeader(QNetworkRequest::ContentTypeHeader, QVariant(
    QString("text/xml")));

   connect(manager, SIGNAL(finished(QNetworkReply*)), this,
    SLOT(serviceRequestFinished(QNetworkReply*)));
    manager->post(request, postData);
}

void LoginWindow::serviceRequestFinished(QNetworkReply* reply){ }

{
    "response": {
    "status": "200",
    "message": "Correct request"
},
"data": [
    [
        {
            "href": "link",
            "text": "string matr"
        },
        {
            "href": "link",
            "text": "string type"
        },
        {
            "href": "link",
            "text": "string name"
        },
        {
            "href": "link",
            "text": "string state"
        }
    ]

]
}