Unit Test Mocking HttpWebRequest

I have a method in a Class and would like build a unit test on it. The method just send data via a HttpWebRequest to a server.

As I would like to test without the server, I found a way here (Is it possible to mock out a .NET HttpWebResponse?) to mock the HttpWebResquest/httpWebResponse. However, I do not know how to make my code call the mocked HttpWebResquest/httpWebResponse object.

Here is my Code:

public long PostRequest(Uri uri,string data)
    {

        try
        {
            HttpWebRequest request = (HttpWebRequest)HttpWebRequest.Create(uri);

            request.Method = "POST";
            request.ContentType = "application/json";

            System.Text.UTF8Encoding encoding = new UTF8Encoding();

            byte[] bytes = encoding.GetBytes(data);



            using (Stream requestStream = request.GetRequestStream())
            {
                //Transmit data
                requestStream.Write(bytes, 0, bytes.Length);
                requestStream.Flush();
                requestStream.Close();
            }



            //Get the Response from the server
            using (HttpWebResponse response = (HttpWebResponse)request.GetResponse())
            {
                if (response.StatusCode != HttpStatusCode.OK && response.StatusCode != HttpStatusCode.NoContent)
                {
                    throw new Exception(String.Format(
                       "Server error (HTTP {0}: {1}).",
                            response.StatusCode,
                                response.StatusDescription));
                }
            }


            return request.ContentLength;
        }
        catch (Exception e)
        {
            throw e;
        }

    }

How could I make my code call the mocked HttpWebResquest/httpWebResponse?