var httpWebRequest = (HttpWebRequest)WebRequest.Create("http://url");

        String username = "abc";
        String password = "123";
        String encoded = Convert.ToBase64String(System.Text.Encoding.GetEncoding("ISO-8859-1").GetBytes(username + ":" + password));
        httpWebRequest.Headers.Add("Authorization", "Basic " + encoded);
        httpWebRequest.ContentType = "application/json";
        httpWebRequest.Method = "POST";

public static void AddAuthorizationHeader(ref HttpWebRequest request)
{
    string username = //load value from web.config
    string password = //load value from web.config
    string encoded = Convert.ToBase64String(System.Text.Encoding.GetEncoding("ISO-8859-1").GetBytes(username + ":" + password));
request.Headers.Add("Authorization", "Basic " + encoded);
}

HttpWebRequest request = new HttpWebRequest();
UtilityClass.AddAuthorizationHeader(ref request);
request.ContentType = "application/json";
request.Method = "POST";