@metrics ,
I understand you are trying to create user in a non-default namespace. The only way to access this namespace for the user is through federation .
Supporting multitenancy with isolated namespaces - Amazon QuickSight [ read through limitations ]
here for example Workshop Studio
Federated users, IAM users and Quick Sight managed users can all be created in secondary namespaces. However, only Federated and IAM users in secondary namespace will be able to access Quick Sight console directly. You can user Quick Sight managed users with secondary namespaces if your use case requires only embedded access. Both dashboard and session/author embedding is possible with Quick Sight managed users in secondary namespaces.
Kind regards,
Koushik