title: CRM 2013 - C# Plugin - Retrieve another entity GUID via lookup
body: <p>I'm creating a plugin that will fire on my custom entity called ContractReview, on the contract review there is a lookup which will hold the sales order associated with it.</p>
  
  <p>In my plugin I can retrieve attributes that are on the form via the lookup. However how do I retrieve the actual GUID of the specific sales order? </p>