Retrieving the GUID of an object in SCSM using PowerShell is sometime a bit challenging. For the WorkItems, this piece of information is not present in any Property available, you have to invoke the get_id method to retrieve it.There is an 'ID' property which is misleading, it contains the number of the request, for example IR123456.
And for some other type of objects, such as the DomainUser or Enumeration objects the GUID is the..... ID property.