Got the $12.50 promotional credit with a $50 gift card purchase, but NOT by manually entering that EGCPRIME22 code.
Instead (as a Prime member), I simply went to the main Amazon gift card page, amazon.com/gift-cards/
At the top is a banner "PRIME DAY EXCLUSIVE, Get up to 25% off on select gift cards or reloads"
Clicked on the banner, and next page was “Get a $12.50 promo credit with a $50+ Amazon eGift Card”, with a little button “Apply code to your account”
(and it shows that code EGCPRIME22 as what is being applied)
Then bought a $50 Amazon card for email delivery to myself, got it, along with a message that the $12.50 promotional credit was coming in 2 days.
I think it may have been also doable with just a $50 reload to my Amazon account; however, I didn’t see the promotional credit mentioned that way.
YMMV as always