I am struggling to configure Facebook app and the apex code to import lead ads. I need to submit the app for an app review for which it asks me the credentials to test the app. Which credentials should I provide for the app review?
Speaking just to the Security Review test credentials issue - you have to provide all credentials the security review team will need to test your app end-to-end.
This usually minimally includes the username/password for the test dev org you setup with your managed package already installed, and any other 3rd party credentials (like a test facebook sign-in, etc).
If you have any questions beyond this on credentials, you can always create a case via the Partner portal to get your specific questions answered, or sign up for Security Review Team office hours