Doubt over HTTP POST

Yes, it is possible in C #. Your code would look like the following sample:

var destinos = "5555-5555\n5555-5556\n5555-9999"; //<-Números que receberão a msg

byte[] buffer= Encoding.ASCII.GetBytes(destinos);

var webReq = (HttpWebRequest) WebRequest.Create("https://xxxxx.com/sendMessageBatch?messageText=mensagemdeteste&key=XXXX");

webReq.Method = "POST";
// webReq.ContentType = "content-type"; <- caso necessário
webReq.ContentLength = buffer.Length;

var reqStream = webReq.GetRequestStream();
reqStream.Write(buffer, 0, buffer.Length);
reqStream.Close();

var webResp = (HttpWebResponse) webReq.GetResponse();

A similar code in jQuery:

$.ajax({
        url: 'https://xxxxx.com/sendMessageBatch?messageText=mensagemdeteste&key=XXXX',
        type: 'POST',
        data: '5555-5555\n5555-5556\n5555-9999',
        success: function(data, textStatus, xhr) {
                //Caso sua operação tenha sido bem-sucedida
            }
        }

Since the release of C # 4.0, it is standard to use HttpClient (part of nuget Microsoft.Net.Http ) for orders web. This class creates a unified API to send / receive HttpRequestMessage s and HttpResponseMessage s.

string[] destinos = {"destino1", "destino2", "destino3"};
var destinosStr = string.Join(Environment.NewLine, destinos);

var content = new StringContent(destinosStr);
var client = new HttpClient();

var response = await client.PostAsync("https://xxxxx.com/sendMessageBatch?messageText=mensagemdeteste&key=XXXX", content);

The API is much clean and easy to use than the WebRequest API. No need to work with buffers and byte arrays, and supports asynchronous tasks.