Xamarin.Forms and IdentityServer3, how to achieve a successful connection?

I've been stuck like 4 days trying to authenticate using IdentityServer3 (OpenID Connect) with no success. I already tried Xamarin.Auth and also IdentityModel with no success.

So far, the only response I got is "invalid_request" each time doing a "correct request" (I mean, when the endpoint successfully redirects to my redirect uri).

Anybody has samples or any approach I can use with Xamarin.Forms (or using dependency service/Custom Render)?