IdentityServer Login on Xamarin Forms app with FaceID/TouchID/Fingerprint/Keychain integration with API

Question

I currently have a Xamarin Forms App that uses IdentityServer4 & .NET Identity for authentication and login-in users to the app, it also integrates with Office356 login. I now have the need to implement FaceID/TouchID so that the user does not have to login every single time the app launches, but it still needs local authentication to prove they are who they say they are. From the research I have done, the username & password get stored in the Keychain OR the Token so that it can be used to login later. I believe I already have a way of accomplishing it using the Token, nevertheless we want to explore all options and double check if there is a better way of doing it by storing the username/password, is there any way to acquire those when using the WebAuth (ASWebAuthenticationSession)? My first issue is that the device browser is the one used to authenticate and I can’t seem to find a way of getting the username/password from there. In the case I was able to, how could I automatically used those to later authenticate (autofill?). Any help is highly appreciated. Thanks in advance