using (WebClient client = new WebClient())
{
    client.Headers.Add("Authorization", "YOUR_ACCESS_KEY");
    var postJson = new JavaScriptSerializer().Serialize(new 
        { 
            messagetext = "This is your code.",
            senderid = "Sendmode",
            recipient = "0870000000" 
        }
    );
    byte[] response =
    client.UploadValues("https://rest.sendmode.com/v2/verify", new NameValueCollection()
    {
        { "message", postJson }
    });
    
    string result = System.Text.Encoding.UTF8.GetString(response);
}