Xamarin.Auth with iTouch - How to?

Hi. are there any PCL examples of using Auth with iTouch (I assume it can only work on iOS)
First your save the USERNAME/PASSWORD using Xamain.Auth. Then how do you retrieve it ,using Xamarin.Forms.Touch?
Once the fingerprint is authenticated you call Auth, and retrieve the USERNAME/PASS and then Login?
It that the 'flow'?
thx in advance