Accessing passkeys on macOS logging into 3rd party apps

I wanted to test out the desktop application provided by Fastmail for macOS.  It of course requires me to log in and I have been using passkeys to login to Fastmail.  Of course 1Password doesn't recognize the app application so macOS tells me I have no passkeys saved for this account and asks me to use a QR code or use a security key.  

I have configured macOS settings in a manner I think 1Password recommends (see screenshot) to support passkeys.  The exception is "Set Up Codes" where my only option is the macOS Passwords app.  Not sure what this last option is really doing, still waiting for Apple to document it better.

What am I missing (if anything)? Using passcodes stored in 1Password when logging into apps simply doesn't seem to work on macOS, and I have had similar issues on iOS.  Must I stop using passkeys for these sites or have a password+2FA and a passkey so I can select one or the other?  I do not want to leave 1Password for Apple's app.