If you have purchased or been gifted a V-Bucks card, you can redeem it on the EpicGames.com website here: https://www.epicgames.com/fortnite/en-US/vbuckscard
Epic Games is the official maker of the Fortnite video game. And that is the official Epic Games website. You don’t redeem these gift cards on any Fortnite domain.
You will need to create an Epic Games account before you can redeem the card. You can read the full terms and conditions via the Epic Games link above.
About V-Bucks Cards
V-Bucks are a virtual currency that can be used in the Fortnite game. Virtual currency is prevalent in gaming nowadays as a more effortless and generally more efficient way to fund the game. V-Bucks cards are supported on Xbox and PlayStation gaming consoles, PC, smartphones, and Nintendo Switch handheld devices.
In conclusion, that is how to redeem your V-Bucks card for Fornite in-game currency.