External Users attributes

I've created a GroupWise external user and I want to use GWAdmin API to
obtain some of its attributes.
I saw that this object doesn't have any value for Distinguished Name and
ObjectID attributes. There are required specific rights to access these
attributes?
There is a certain attribute that uniquely identifies a GroupWise
External User? ( something like GUID)? I saw that the object has
External GUID attribute but I don't know how to access it using GW Admin
API.

Any suggestion is useful.

Thanks in advance.