REST API. How to get an attribute of a user

With
https://<server>:9710/gwadmin-service/domains/<domain>/postoffices/<postoffice/users/testuser
I get a whole bunch of information of the user testuser. Good.
But what I really would like as an output is only one attribute of this user, e.g. visibility.

To append ?attrs=visibility to the above URL does not work.

Also with
https://<server>:9710/gwadmin-service/diagnostics/object/
USER.<Domain>.<Postoffice>.testUser
I have no luck.

What can be done to retrieve only on attribute?

Best regards, Uwe.