For the iOS app. There is no folder to choose - the container is this app's own, fixed by its identifier - so attaching it is one tap rather than a picker.
Nothing to configure. No fields, no credentials, no CORS. iCloud Drive turned on for the device, and turned on for this app specifically, which iOS asks about the first time it is opened.
The iOS app only. Not a browser, not the macOS desktop app - both of those reach a synced iCloud Drive folder through a folder on this computer instead, if you want the same drafts visible there too.
Unlike a picked folder, there is nothing to remember or re-grant: the path is asked for fresh every time the app opens, never cached from last time. Sign out of iCloud, or turn it off for this app, and that is caught on the spot rather than found out later.
It is the same storage_list/storage_read/storage_write/storage_remove machinery a picked folder uses underneath - iCloud Drive only changes where the root points, not how anything past it behaves.