Manipulating files outside of sandbox

First, we need to ask for the user's permission to access the folder, but for that, we need an NSSavePanel:

let op = NSSavePanel()
op.message = "Descriptive message here"
op.canCreateDirectories    = false
op.canChooseDirectories    = false
op.showsHiddenFiles        = false
op.prompt                  = "Allow"
op.title                   = "Allow access"
op.isExtensionHidden         = true
op.directoryURL            = URL(string: "/path/to/folder")

// Depending on your purpose, you might need these to true
op.allowsMultipleSelection = false
op.canChooseFiles          = false

We then check if the panel's URL matches the one we need, and save a secure scoped bookmark in user defaults, for example:

guard op.runModal() == .OK && openPanel.URL == requiredURL else {
	return
}

guard
	let bookmarkData = try? requiredURL.bookmarkData(
    	options: [.withSecurityScope],
		includingResourceValuesForKeys: nil,
		relativeTo: nil),
	bookmarkData != nil
else { return }
UserDefaults.standard.set(bookmarkData, forKey: requiredURLKey)

From now on, when we need access to this location, we can just resolve the data saved back into an NSURL, and use it; we just need to check if the bookmark staled in the meantime:

var staleBookmark = false
guard
	let bookmarkData = UserDefaults.standard.data(forKey: requiredURLKey),
	let url = try? URL(resolvingBookmarkData: bookmarkData,
		options: [.withSecurityScope],
		relativeTo: nil,
		bookmarkDataIsStale: &staleBookmark)
else { return }
		
guard !staleBookmark else { return }

// [...] Use the URL.