FIDO2 is not something you can "paste on" it has to be built-in to the login feature, but I guess for things like Wordpress there are hooks you can use.

I know of no plugins or APIs you can use to add it, I guess if they appear they will be listed on Yubicos list [0] or on Github [1]

[0] https://developers.yubico.com/Software_Projects/FIDO_U2F/U2F... [1] https://github.com/search?q=FIDO2

Important: U2F cannot be used as the only authentication method, unlike FIDO2 which fully replace password & 2-factor.