HttpClient client = new HttpClient();
string key = "YOUR_API_KEY";
string user = "YOUR_PHONE_NUMBER";
string pass = "YOUR_PASSWORD";
string sender = "SENDER";
string recipient = "0512345678; 0512345678"; // Numbers must be separated with ;
string msg = "SMS4FREE"; // can be anything

var requestObject = new
{
key,
user,
pass,
sender,
recipient,
msg
};

var dataAsJson = JsonConvert.SerializeObject(requestObject);
var buffer = Encoding.UTF8.GetBytes(dataAsJson);

var byteContent = new ByteArrayContent (buffer);
byteContent. Headers.ContentType = new MediaTypeHeaderValue("application/json");

var response = client.PostAsync("https://api.sms4free.co.il/ApiSMS/Send SMS", byteContent).Result;
var responseString = response.Content.ReadAsStringAsync().Result;
Console.WriteLine(responseString); // Gives you how many recipients the message was sent to