Sign in to the your Vaultwarden account, then expand the "Settings" dropdown (in the left hand column of the page), then select the option "Security";
The page will update to list the security settings, select the sub-menu option "Two-step login", then against the provider "Fido Webauthn" click
;

A window will open titled "Two-step login", at the "Master password" prompt enter you password then click 

You will now be asked to provide a name for your Fido Key;

Insert you Fido2 key in an available USB port, then at the prompt "Name", provide a suitable name for your key then click
;

You will now be presented with a list of possible passkeys, select "Security Key";

You will mow be asked to touch the button on your key (if your model has a fingerprint reader you may be asked to provide a finger swipe instead);

Provided you press the button on the Fido2 key in the allowed time, the passkey details will be stored on your Fido2 key, and you will be asked to confirm fido key activation;

Click
, and registration and activation of the Fido2 key will be completed and you will be presented with the following confirmation;

Your Fido key is now ready to be used as an authentication factor next time you log in to Vaultwarden.