For anyone else struggling with this I did find an answer on stackoverflow here: https://techcommunity.microsoft.com/t5/microsoft-teams-blog/announcing-general-availability-of-the-new-microsoft-teams-app/bc-p/3947306/highlight/true#M13303
There is an update in the comments of the New Teams Desktop GA announcement here https://techcommunity.microsoft.com/t5/microsoft-teams-blog/announcing-general-availability-of-the-new-microsoft-teams-app/bc
- Right click the Teams client icon in system tray to see the Engineering Tools entry point.
- To enable engineering tool in New Teams client
- Create a configuration.json file under: %localappdata%\Packages\MSTeams_8wekyb3d8bbwe\LocalCache\Microsoft\MSTeams
- In the file add: {"core/devMenuEnabled": true}
- Restart the client.