Returns the Id of the default server if one was assigned to the service. 
   Namespace:   Autodesk.Revit.DB.ExternalService  
  Assembly:   RevitAPI  (in RevitAPI.dll) Version: 2015.0.0.0 (2015.0.0.0) 
  Since:  2013 
Syntax
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Return Value
The GUID of the default server, or an invalid GUID if there is none assigned.Remarks
 Only mandatory services have default servers assigned. Only built-in service can currently be mandatory. All third-party services are optional and cannot (amd may not) have default servers.