I see the (probably) correct property, but I cannot set it since it's private...
The SDK targets the most recent, non-preview API version (today it's 2015-02-28). Features that are in preview can be used through Azure Search REST API. What are you trying to do?
Yesterday we released the 1.1.0 version of our SDK that includes support for Lucene query language. You can read about the roadmap for our SDK here: https://azure.microsoft.com/en-us/blog/azure-search-sdk-roadmap/.