An example ASP.NET 6 (Preview 4 with Minimal API) web server for use to verify AdMob Rewarded Ad callback system named SSV (Server Side Verification)
Listening :88 for HTTP and :444 for HTTPS because you may want to create another ASP.NET instance to your domain for to serve default website requests.
You can create an "Rewarded Ad" and enable "Server Side Verification" for it at AdMob Console.
In test screen, you can call your server like https://www.YOURWEBSITE:444/ to try and verify callback URL.
Don't forget the fill "user_id" parameter.
Required Visual Studio 16.11 Preview 1 to publish at least.
Required .NET 6 Preview 4 to build at least.
Change appsettings.json file to use correct Path and Password for your domain's SSL Key. AdMob requires HTTPS connection.