I'm not sure if the process will involve a code or not. I don't see anything about that on this dual entitlement FAQ page:
https://www.ea.com/games/madden-nfl/...ment-info-faq?
The thing that is mentioned (several times) is logging in with the same account that purchased the current-gen game.
Plus, I'm pretty sure they can do it without a code - No Man's Sky PS5 downloaded just fine with nothing else other than using the account I purchased the PS4 version with. I'd guess that the only way a code would be involved is EA needing to send you one in case you're going to a discless PS5 after owning the PS4 disc.