@carltonb I have just got this working. You need to:
Follow the instructions at this link to get values for YUBICO_CLIENT_ID and YUBICO_SECRET_KEY.
In UNRAID - open the settings for the bitwardenrs Docker container
At the bottom click on "Add another Path, Port, Variable, Label or Device"
Set Config Type = Variable, Name = YUBICO_CLIENT_ID, Key = YUBICO_CLIENT_ID, Value = <Your Yubico Client ID>
Click Add
Add another variable as above for YUBICO_SECRET_KEY
Click APPLY to restart the Docker container
That should be it - now you can configure the Yubikey in the Bitwarden Settings.