protected:
    virtual bool nativeEvent(const QByteArray &eventType, void *message, long *result);

bool MainWindow::nativeEvent(const QByteArray &eventType, void *message, long *result)
{
    UINT MyMessage = RegisterWindowMessage(L"MY_MESSAGE");
    UINT msg = *(reinterpret_cast<UINT*>(message));
    if(msg == MyMessage)
    {
        *result = 0;
        MessageBox(0, L"Got", L"MyMessage", MB_OK);
        return true;
    }
}

void MainWindow::on_pushButtonSendMessage_clicked()
{
    HWND h = FindWindow(0, L"Child");
    if(h!=0)
    {
        UINT MyMessage=::RegisterWindowMessage(L"MY_MESSAGE");
        if (MyMessage==0)
        {
            MessageBox(0, L"Impossible register message", L"", MB_OK);
            return;
        }
        SendMessage(h, MyMessage, 1, 2);
    }
    else MessageBox(0, L"Can not find you window", L"", MB_OK);
}

if (!message)
 {MessageBox(0, L"info", L"nullPointer", MB_OK};
return;