using System.Net;
using System.Text;

var request = WebRequest.Create("https://onesignal.com/api/v1/notifications") as HttpWebRequest;

request.KeepAlive = true;
request.Method = "POST";
request.ContentType = "application/json; charset=utf-8";

request.Headers.Add("authorization", "Basic NGEwMGZmMjItY2NkNy0xMWUzLTk5ZDUtMDAwYzI5NDBlNjJj"); // Seu token do OneSignal, você pega lá em Keys & Ids

var serializer = new JavaScriptSerializer();
var obj = new { app_id = "5eb5a37e-b458-11e3-ac11-000c2940e62c", // ID do seu OneSignal, pega em Keys & Ids também
               contents = new { en = "MENSAGEM DA NOTIFICAÇÃO" }, // Mensagem que será enviada para o usuários
               included_segments = new string[] {"All"} }; // Envia para todos os celulares que possuem o aplicaivo autenticado
var param = serializer.Serialize(obj);
byte[] byteArray = Encoding.UTF8.GetBytes(param);

string responseContent = null;

try {
    using (var writer = request.GetRequestStream()) {
        writer.Write(byteArray, 0, byteArray.Length);
    }

    using (var response = request.GetResponse() as HttpWebResponse) {
        using (var reader = new StreamReader(response.GetResponseStream())) {
            responseContent = reader.ReadToEnd();
        }
    }
}
catch (WebException ex) {
    System.Diagnostics.Debug.WriteLine(ex.Message);
    System.Diagnostics.Debug.WriteLine(new StreamReader(ex.Response.GetResponseStream()).ReadToEnd());
}

System.Diagnostics.Debug.WriteLine(responseContent);