Fortnite Save the World Unlock?

I would like to know if I would now buy an unlock code for save the world from Instant Gaming or MMOGA, where do I have to enter the code then simply redeem it at epic games and then it is unlocked or only possible if you are playing the game for the first time?

You go into the game, then save the world, then it says Buy and Redeem Code…

If you click on save the world only the info text comes up and if you click on more details you can only choose between standard founder package and deluxe and pay directly nowhere is something to enter.

Yes, I play on the PC, so I don't know if you have to enter it in the epic launcher or where else…