Redeem V-Bucks Code PC?

I've received a code twice for each 600 V-Bucks through an app but in Fortnite itself you can redeem a code only from 1,000 V-Bucks. Does anyone know and knows how or where I have to insert these two codes so that the V-Bucks get on my Fortnite account?

Not sure on the Epic Games page?

You can't get through an app V-Bucks, which can only be purchased at Epic itself.

And if it does, then it's associated with a lot of risks like Abofallen and Co.