(Transaction) LMDB: How to use OTHER "Namespace" than "active"

Dear colleagues!

In SLD I have the possibility to setup a namespace additionally to the standard namespace "sld/actice" -
for testing purposes, for example.

My question:
In (TX) LMDB I've found a "Namespace" input field as well - right upper corner,
BUT I'm missing a possibility to CREATE a "Namespace" different from "active" -
cound you guide me to do so?

Kind regards,
Peter.