Configuring your application to start leveraging these features happens at with your WebHostBuilder, since the web server negotiates all of the incoming HTTP requests and determines support for various protocols. Once that's done, though, your clients will begin seeing the benefits of the protocol extensions without any additional change in your code to support it.
Unfortunately, at the time of writing, Kestrel doesn't support some features, such as server push and stream prioritization. So, while the client can send a prioritization tag with its requests, Kestrel simply won't act on that request. And one other caveat to HTTP/2 support is that your hosting environment's native cryptography library must support Application ...