A General purpose rest ApiClient write in C# language for the OneSignal API
PM> Install-Package OneSignal.CSharp.SDK
var client = new OneSignalClient("ZWM3YThlMmQtMzY1NC00ODI1LTlkYjMtMTk3MzI2OTQzMjVh");
var options = new NotificationCreateOptions();
options.AppId = "089e4845-9849-4157-8b29-71e174af3abf";
options.IncludedSegments = new List<string> { "All" };
options.Contents.Add(LanguageCodes.English, "Hello world!");
client.Notifications.Create(options);
[OneSignal Server API] (https://documentation.onesignal.com/reference)