This is an odd one. I visited
this app page on Amazon's App Store and there's a little box that says:
"Download now" links to the
Hay Day game page. (That's a direct link, not the link provided on the first page, in case that one has user specific codes in it.)
I've searched about the promo and only really found someone who said they got an E-mail about it, and were asking how to tell for sure they have the credit. So this
may be a targeted offer. Or it may be available to everyone.
If you can see the little box telling you about the promo, you probably qualify for it.
I tried downloading and running the game, and I can't see any way to tell if I have the credit or not. Maybe someone else will have better luck.