using (WebClient client = new WebClient())
{
    client.Headers.Add("Authorization", "YOUR_ACCESS_KEY");
    string[] recipients = new string[1];
    recipients[0] = "0870000000

    string message = "Hello World";
    var postJson = new JavaScriptSerializer().Serialize(new
    { 
        messagetext = message, 
        senderid = "Sendmode", 
        recipients = recipients 
    });
    byte[] response =
    client.UploadValues("https://rest.sendmode.com/v2/send", new NameValueCollection()
    {
        { "message", postJson }
    });

    string result = System.Text.Encoding.UTF8.GetString(response);
    
}