Where is the private forum for the Universal App Quick Start Program / Developer Transition Kit (DTK)?
From an Apple ID that has been accepted to the Universal App Quick Start Program, access the Apple Developer Forums.
Click your profile icon at the top right of the screen:



You should see a link to the private forum:

Developer Forums
Universal App Quick Start

Which links to the forum/tag for the UAQSP/DTK, located here:
https://developer.apple.com/forums/tags/universal-app-quick-start

Universal App Quick Start
Private forums for program members


Notably, this Universal App Quick Start tag is not included in the list of all forum tags. So it's rather hard to find.