Xamarin.Forms working with 3rd party ASP.net C# SDK

I'm trying to work with a REST service using a 3rd party ASP.net C# SDK, but I can't add it as a reference to the PCL project and thus can't add the correct using statements. I'm new to development in general and really new to working with Portable Class Libraries. Is there a method for working with 3rd party SDKs in Xamarin.Forms or do I need to look into adding some of the existing files from the SDK to the PCL project. The files would be a helper class and some services.

Any help/guidance is much appreciated.

Thanks!