iOS 13 - How to login in in-app purchase Sandbox account?

In order to test my in-app purchases with iOS 13 I have updates one of my test devices to iOS 13.1 beta.

On iOS 12 and before I there was a special Sandbox login in Settings/iTunes & App Store/Sandbox Account:

However, after the update to iOS 13 this section is missing. I tried to follow the instruction in the answer to a similar problem on iOS 12 but nothing worked. I completely rest the device and logged out both at Settings/iTunes & App Store and at Settings/Apple ID, so currently the device is not connected to any account (real or sandbox). At least as far as I can tell.

I have re-installed my app on the devices using Xcode 11 beta and tried to perform an in-app purchase. The store shows a login prompt which shows that this is a Sandbox purchase. However, the prompt only ask for the password, not for a username or Apple ID. So I have no idea which account should be used here.

When using the password of a newly created test user account the password is not known. I can use the password of my real Apple ID account which was used during the device setup and was than disconnected.

So, how to connect to a specific Sandbox account in iOS 13?

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