How do I install a folder under the current user's Documents?
I don't see it as an option under the Payload tab.
Also looked at the documentation and can't find anything.
Gave up trying to do that years ago.
Make the packager install your app only.
Usually it gets put into Applications, but if the user has moved it, a subsequent install will try to replace it where it was moved to.
Your app should then create data that it requires when it starts, if the data does not already exist , maybe by unpacking a zip file or duplicating items from the resources folder.
That unpacking can occur into Specialfolder.applicationData.child("yourappname")
or into specialfolder.documents.child("yourappname") if you wish
That means that if another user runs the app, they get their own set of support files in their own folders.
ApplicationData is the recommended place, although like you, I create files in Documents, because they are easier for the user to get at them. (I want them to be able to do that) .. ApplicationData is hidden by default most of the time.