Your bundle identifier is the same as your app ID. It is a reverse domain name, such as com.mysite.app.
There is a default app ID created for you, but you may change it at any time. You can change it in the app customizer under the Settings tab:
This ID needs to match the ID you use for your app store listings and iOS certificates. In your Apple Developer account, you create this app ID under Certificates, Identifiers & Profiles => App IDs.