Passkey will show once you have registered one for a given website. Usually, when registering for a website that offers passkey login, it will offer you to register a passkey when creating your account, then you can unlock your vault from the bitwarden extension and register the passkey. Otherwise for an already existing account, you can go to your account/security settings for this website, if they offer passkey login, it will be in the same section as for security keys (yubikey, solokeys), the registration process is similar to registering a hardware security key, only bitwarden will act as the passkey. You can see it as a de-materialized security key.