Disable the focus system for a MacCatalyst app

I'm building a MacCatalyst app, where the Tab key is used for a special functionality within the app. For that, I inserted a UIKeyCommand in the main menu for "\t".

When I press Tab, the default functionality takes over, and the focus moves to a button in the toolbar and stays there, which I don't want. As a result, the action of my key command is not called. I set tabKeyCommand.wantsPriorityOverSystemBehavior = true, but it makes no difference.

The root view controller of my app is an UISplitViewController. I overloaded 'shouldUpdateFocus(in:)' and always return false hoping that this will disable moving the focus for the whole app, but it's not working. It looks like it's more difficult then needed.

How can I instruct the system to not interfere with the Tab key?