Log in to your Yahoo account by supplying your username and password details, but once these have been supplied you will be presented with an additional popup asking for a passkey;
Insert your Fido key into an available USB port, then click "Security key";
At the prompt "Security Key PIN", enter the PIN code, then click 
.
(in the case of a Fido key with a fingerprint reader you will need to swipe your finger on the key);
You will now be asked to touch the Fido2 key (in the case of a Fido key with a fingerprint reader you will need to swipe your finger on the key);
Once you have touched the key you will then be logged in to your Yahoo account.