You can try the following way for getting the AdminDomainRef .
For eg.,
ObjectIdentifier oid = ObjectIdentifier.newObjectIdentifier("wt.projmgmt.admin.Project2:xxxxxx");
Project2 project = (Project2) PersistenceHelper.manager.refresh(oid);
WTContainerRef project_ref = WTContainerRef.newWTContainerRef(oid);
AdminDomainRef domain_ref = project.getDefaultDomainReference();