There's an issue "Implementation of iOS Promotional Offers #92231"
which is closed by a pull request in plugins in-app-purchase
but the implementation is not 100% complete and there is no way to get the promotional offer to work.

I've searched the net for example of using the newly added SKPaymentDiscountWrapper class (introduced in the above mentioned pull request) , but did not found anything. After trying to make it work on my own, I noticed several missing pieces which I will add in a new pull request.