How do I receive automatic notifications from PagSeguro?

I am implementing in the company where I work, the payment with the Pagseguro. So far, I've been able to generate a payment request and also receive the transaction code on my page. The problem is to receive the notification code via URL, I already set the return URL in the API, but I still do not receive it. Could anyone help?

To receive the code via QueryString, use the code below.

Thank you in advance.

protected void Page_Load(object sender, EventArgs e)
    {
        string notificationCode = string.Empty;
        string notificationType = string.Empty;

        //Pega dados vindos do post
        notificationCode = Request.Form["notificationCode"];
        notificationType = Request.Form["notificationType"];

    }