Update for other users who might stumble upon this post: It turned out that Xavier1978 was using "netstandard2.0" DLLs on Xamarin.Android, which is not a supported platform for these yet - see our platform support chart for details. For now "xamarin.android" DLLs should be used on this platform.
For client authentication with a certificate in .P12/.PFX file, see the instructions here: Client certificate authentication.