First, read the documentation:

https://developer.apple.com/documentation/bundleresources/privacy_manifest_files/describing_use_of_required_reason_api

The section for UserDefaults is the last one (saving the best for last, of course!)

You'll need to add a new file to your Xcode project. Make sure the PrivacyInfo.xcprivacy file is included in the target (so that it can be extracted after you upload to the App Store).

The documentation here explains how to create the file:

https://developer.apple.com/documentation/bundleresources/privacy_manifest_files#4284009

The format of the XML in the file isn't obvious. There's a lot of talk about keys for types and description, but the hierarchy wasn't clear to me.

Here's where I landed on the file: https://gist.github.com/chockenberry/2c1c829dba9c7f34c9a7e8e04335be42

NOTE: This file assumes that you only use UserDefaults in the app. If you're using an app group for an extension or whatever, the “CA92.1" won't fly. Read the docs I mentioned earlier!

Also note that this PrivacyInfo.xcprivacy is only for UserDefaults. You may (and probably!) have other APIs you need to declare.

This is a nice little shell script that helps you find the culprits (before the App Store can complain about them):

https://github.com/Wooder/ios_17_required_reason_api_scanner

It sure would have been nice for Apple to document the most widespread use case for privacy.

The motivation here is excellent: it's an effort to reduce the fingerprinting that shithead developers like to do. I'm all for that.

But making me work to figure it all out? And making every other developer who's used UserDefaults do the same thing?

Come on, that's low hanging fruit!