Create a top level category within Kunena > Categories to hold all of GJs group forums. Next in CB GroupJive Forums > Parameters > General set "Forum" to "Kunena" and "Category" to that top level category you created. That's all you need to do. If you've existing GJ groups re-save them for it to create their categories in Kunena and all future new groups will do so automatically on create.

Kunena v6.0.0-BETA7-DEV_2022-01-07

None of this is tested with Kunena 6.0.0. We don't test or integrate with beta releases. I just don't have that kind of time. Once it's stable we'll check our integrations to make sure they're still functional. For now I've reconfirmed this is working fine on Kunena 5.2.8.

We'll retest the implementation once stable is available. I just don't have the time to deal with testing integrations against betas. The dropdown being empty suggests possible API changes for the categories API.