How can this be used with HMAC?

[srfsh]

Excuse my ignored question but I just couldn't figure it out. From what I can tell, the cryptographic utilities of the standard library of dotnet appears to have an HMAC interface, but only a few solid implementations without much a way to derive yours using a custom hashing function. I can't quite tell why. I tried to look up a generic HMAC library on GitHub but I also failed to find one there. Is there something I am missing? Why is this appears to be quite difficult?

P.S.: I'm not a well-seasoned C# developer.