Use this implementation instead Easy Forms Auth. There is also a nice tutorial here on how it works.

This is a login sample which uses Xamarin.Auth and implements custom renderers to create a login page. Code is fairly straight forward. You are welcome to use the code as a starting point for your project. In each of the customRenderers you will need to change the OAuth Credentials to reflect your credentials. Code has comments to help you out. Windows Phone token storage is still pending. Be sure to give a star if it works and helps you :)