Sample app that shows how to do Authentication/Authorisation with ASP.NET Web API Identity and Xamarin.
OAuth Overview with ASP.NET Identity : https://youtu.be/UToBKlnig4M
Signup user to ASP.NET Identity from Xamarin Forms : https://youtu.be/t0v4srA4NhA
Login user to ASP.NET Identity from Xamarin Forms : https://youtu.be/Nkccb9ODr_A
Invoke protected web services : https://youtu.be/T4FI1LgCqn0
Register Access Token in Xamarin Forms : https://youtu.be/zqS9EjuekDs
Create authenticated HTTP Post request : https://youtu.be/Mg4qvVBXfrI
Create authenticated HTTP Delete request : https://youtu.be/Mx68Nk-XdO8