Android- how to test a promo code in debug mode or locally?

Question

Another Iap mistery from google or I dont get something here. Google says that test your promo code in the docu. But how can i even test it? When I try to test promo code using licence tester's email address. Google is only displaying "test credit card" as the payment option? Screen doesnt have any "Enter Promo code" option like in Production! Basically this is the only screen you get

I want to debug the response and everything. Ok I can put some logging also without debugging but when I dont use "license tester", Google is returning "this item is not available for purchase" response. So only way remains is that i can test promo codes in production.

Do i understand something wrong here or is there any other way to achieve this? I am on the billing library 4.0