How create request and redirect client to payment gateway master card in C# using ASP.NET MVC 5?

This may be code I tried.

I am trying to redirect the client to the payment gateway after payment is done it returns back by return URL.

But I did not figure out how:

[HttpPost]
public ActionResult PaymentGateway()
{
    var httpWebRequest = (HttpWebRequest)WebRequest.Create("https://abc.gateway.mastercard.com/api/rest/version/6/merchant/xxx/order/1/transaction/1");
httpWebRequest.Credentials = CredentialCache.DefaultCredentials;

    string credentials = Convert.ToBase64String(Encoding.ASCII.GetBytes("xxxx:xxxx"));

    httpWebRequest.Headers.Add("Authorization", "Basic " + credentials);
    httpWebRequest.ContentType = "application/json";
    httpWebRequest.Method = "POST";

    using (var streamWriter = new StreamWriter(httpWebRequest.GetRequestStream()))
    {
        string json = "{\"apiOperation\":\"CAPTURE\"," +
                              "\"order.amount\":\"100.000\"}" +
                              "\"order.currency\":\"USD\"}" +
                              "\"browserPayment.redirectUrl\":\"https:\\example.com\callback\"}";

        streamWriter.Write(json);
    }

    // how send request ?
    return ??? ;
}