private const string AllScopes = "general_info contacts finances events event_registrations account membership_levels settings";

var authData = string.Format("granttype={0}&scope={1}", "clientcredentials",AllScopes);

        var response = System.Net.WebRequest.Create(Common.OAuthServiceUrl)
            .SetBasicAuth("APIKEY", "........YOUR API KEY HERE......")
            .SetData(authData)
            .GetResponse();

Extension Methods:

    public static WebRequest SetData(this WebRequest request, string data, string contentType = MimeFormUrlEncoded, string method = "POST")
    {
        request.Method = method;
        request.ContentType = contentType;
        byte[] sentData = Encoding.UTF8.GetBytes(data);

        request.ContentLength = sentData.Length;
        Stream sendStream = request.GetRequestStream();
        sendStream.Write(sentData, 0, sentData.Length);
        sendStream.Close();

        return request;
    }

    public static WebRequest SetBasicAuth(this WebRequest request, string userName, string password)
    {
        var credentials = string.Format("{0}:{1}", userName, password);
        var encoded = System.Convert.ToBase64String(System.Text.Encoding.UTF8.GetBytes(credentials));
        request.Headers.Add("Authorization", "Basic " + encoded);
        return request;
    }