I was looking for the same thing.
I ran into this topic on this forum: Store file to iCloud
It is not covering the selecting a file part. But it gives some insight to saving files to iCloud.
I hope there will be a SpecialFolder.iCloudFolder someday. Maybe there is even a feature request already. If not, somebody should definitely write a request.
Also, an OpenFileDialog might be useful for iOS apps, to access iCloud files and files stored on a connected USB drive.