Requesting the Strava authentication API using POST c# keeps failing c#

Question

I am trying to access the Strava authentication API:

https://developers.strava.com/docs/authentication/

But I keep getting an "An existing connection was forcibly closed by the remote host" error, regardless of how I post my request server side using c#

I have tried both HttpClient and HttpWebRequest

var request = (HttpWebRequest)WebRequest.Create("https://www.strava.com/oauth/token");

var data = Encoding.UTF8.GetBytes("client_id=someid&client_secret=somesecret&refresh_token=sometoken&grant_type=");

request.Method = "POST";
request.ContentType = "application/x-www-form-urlencoded";
request.ContentLength = data.Length;

using (var stream = request.GetRequestStream())
{
    stream.Write(data, 0, data.Length);
}

var response = (HttpWebResponse)request.GetResponse();
var responseString = new StreamReader(response.GetResponseStream()).ReadToEnd();

It works perfectly when using JavaScript, jQuery or Postman.

function doPost() {
            $.post("https://www.strava.com/oauth/token",
                {
                    client_id: "someid",
                    client_secret: "somesecret",
                    refresh_token: "sometoken",
                    grant_type: "refresh_token"
                },
                function (data, status) {
                    // done
                });
        }

Any help pointing me in the right direction will be greatly appreciated

Thnx