Unique identifier of an external service. 
   Namespace:   Autodesk.Revit.DB.ExternalService  
  Assembly:   RevitAPI  (in RevitAPI.dll) Version: 2015.0.0.0 (2015.0.0.0) 
  Since:  2013 
Syntax
Remarks
 Each external service must have a unique ExternalServiceId. The Id can be registered with Revit for servers to easily find it. Unique ExternalServiceId should be created using GUID generation tool. ExternalServiceId can later be used to access registered external services.