If you are having trouble using a virtual hardware key in AutoFill mode it could be because:
It is your first time using this database and you are using Face/Touch ID or PIN Code
You are using the wrong Virtual Hardware Key
The first time you use a Virtual Hardware Key in AutoFill you cannot also use Face/Touch ID or a PIN Code. This is a limitation we are working on. For the moment you need to work around this by:
Go into the main Strongbox app, unlocking your database and then disabling Face/Touch ID and/or PIN Code.
Return to Safari/Third Party App and tap on the "Key icon > Strongbox" instead of the QuickType "suggestion"
Once you see your databases list, tap on the correct database and manually enter your credentials including setting the correct Virtual Hardware Key. This should unlock for you.
Now that this is setup you can re-enable Face/Touch ID or PIN Code in the main app.