If by "an application that requires prior onboarding by the company" you mean an application that requires that the user have a separate relationship with the company in order to use the app then.....
All apps must do something, not much, but something. So your app must open and tell the user that the app is
'for customers of MyBigBank, the finest bank in all of East Country. If they you not yet a customer they can join at any branch or call us at 800-555-1212. To log-in enter your username and password here:.....'
It may be possible to get Apple to approve an app that requires the log-in immediately. But why bother? Show the unaffiliated user how to join your group or show them your products.