How to use OST keys?

I have been trying to use the kingdom and castles OST key, but I have no idea were or how.
I have tried adding it to steam but it says its not a product.
I tried looking in game were to add it, but I can’t find it.

I have had the same problem before with another game and it’s OST key.
I honestly don’t even know if the OST key should be used for the game or somewhere else.

If anyone could help me then I would be really grateful.
Thanks beforehand.

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I believe the key is for bandcamp


You can redeem the key here: Aether