You can save it to a subdirectory folder named with your bundleID inside the application support folder or you can also save it to the preferences folder located inside the library directory:
let preferencesDirectoryURL = FileManager.default.urls(for: .libraryDirectory, in: .userDomainMask).first!.appendingPathComponent("Preferences", isDirectory: true)
let fileURL = preferencesDirectoryURL.appendingPathComponent("fileName.plist")
print(fileURL.path) // "/var/folders/.../Library/Preferences/fileName.plist
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