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

Here’s what worked for me (on a dedicated development iPhone 6s running iOS 13.3).

1. Sign out of iTunes & App Stores
2. Sign out of iCloud
3. Run app on device, and click to purchase an item.
4. When prompted for an ID/Password, enter the Sandbox Tester credentials.

It took entering the credentials a couple of times, but the In App purchase dialog alert did say the environment was Sandbox.

After it “made good” (that is, stopped asking me to log in and processed the SKPaymentTransation), I was able to go to iTunes & App Stores in the General Settings, scrolled to the bottom, and now the Sandbox Account appears with the tester account showing.

Now when I run the app and click the button to purchase content, it just asks for the tester password.

Hopefully this helps others.