The Bundle ID is entirely created by the developer of the app. They just need to put it in reverse domain format. What I normally do when I can’t figure out the exact Bundle ID for an app is just do a web search with the app name and “bundle ID”, and you usually find a website with basic info about the app which includes the ID.
Another option is to just contact the developer of the app. You should also mention to them how iTunes File sharing should be enabled so that people can access their files better in iOS 8.3 or later.