Darvos,
Sign in to Apple Developer (developer.apple.com) and go to your Certificates, Identifiers & Profiles. Go to Identifiers, and then you should have the app ID you created for your game. It will have its name on the left, and the ID on the right will be what you put in “Bundle Identifier.”
If you haven’t set any of that up, then you will need to join apple developer before you can get this ID etc. Here is the UE4 documentation that I used to help, since I just did all this too: https://docs.unrealengine.com/latest/INT/Platforms/iOS/QuickStart/5/index.html