A General purpose rest ApiClient write in C# language for the OneSignal API
PM> Install-Package OneSignal.CSharp.SDK.NetStandard
var client = new OneSignalClient($"{RestApiKey}");
var options = new NotificationCreateOptions();
options.AppId = $"{AppId}";
options.IncludedSegments = new List<string> { "Segment1" };
options.IncludeExternalUserIds = new List<string> { "CustomId1", "CustomId2" };
options.Contents.Add(LanguageCodes.English, "Hello world!");
client.Notifications.Create(options);
[OneSignal Server API] (https://documentation.onesignal.com/reference)