Consume an https service on asp.net c #

I would like you to help me I am trying to consume a service to my application asp.net c # a that the service I want to consume is .ashx

but when I add the service reference I get an error like the image I attached.

I would like you to help me or because it does not allow me to consume the service I want to consume is this:

link

protected void Page_Load(object sender, EventArgs e)
        {  
            string URLString = "https://robinacademia.com/p.ashx?o=63&e=2&f=pb&r=REQUEST_SESSION_ID&t=TRANSACTION_ID";

            XmlTextReader reader = new XmlTextReader(URLString);
            string resultado = "";
            while (reader.Read())
            {
                switch (reader.NodeType)
                {
                    case XmlNodeType.Element: // El nodo es un elemento.
                        resultado += "<" + reader.Name;

                        while (reader.MoveToNextAttribute()) // Lee los atributos.
                            Console.Write(" " + reader.Name + "='" + reader.Value + "'");
                        resultado += ">";
                        resultado += "\n";
                        break;
                    case XmlNodeType.Text: //Muestra el texto en cada elemento.
                        resultado += reader.Value;
                        if (resultado == "<result>\n<code>\n1")
                        {
                            txtnumero1.Text = reader.Value;
                        }
                        else
                        {
                            txtnumero2.Text = reader.Value;
                        }
                        break;
                    case XmlNodeType.EndElement: //Muestra el final del elemento.
                        resultado += "</" + reader.Name;
                        resultado += ">";
                        break;
                }
            }
        }