Header and Sidebar frames - how to activate?

Hi there,
I am overlooking something but I don's see how to activate the Header and/or Sideframe frames as in https://docs.retool.com/docs/app-editor#header-frame
I am only seeing the Main frame and cannot find how to activate the other frames.


Thanks for your help.

gr. Erik

Extra info: I have also found this page and information Introducing the Sidebar Frame: Create intuitive interfaces for complex apps
And when I import the demo app, I see the header and sidebar. But I have no idea on how I can activate on my own apps. If I create a new app, I only have a Main frame.

But, after clicking on all possible options and places, I have found the place where I can activate the Header/Sidebar frames.