If your app has a login functionality, you need to create and supply a test/demo account for Apple during review. This account is for App store review purpose only. You can delete this account once the app is approved.
Under iTunes Connect Developer Guide check Providing App Review Information section which says,
Use the App Review Information section to provide contact, review notes, and demo account information so that the App Review team can review your app. If your app requires specific settings, user registrations, or account information before you submit it to the App Store, be sure to include that information in this section.
Also refer similar questions: iPhone App Store Release Question and Demo account in itunesconnect in sms-based app
Hope that helps!