Web Push Notification - ASP.NET

I want to make a push notification web app. I do it: https://developers.google.com/web/fundamentals/codelabs/push-notifications/

But when I reach to back-end side I can't do it. The library is this: https://github.com/web-push-libs/web-push-csharp

It seems complex to me. Please Help me. Thank You in Advance!