In order to activate a key on Minecraft.net, you will need to either create a new account on Minecraft.net or log in there using an existing Microsoft account. Then please simply follow these steps:

1. First, claim the purchased product on your Kinguin account. 
2. Log in to your account.
3. Go here to redeem your key.
4. Paste your code to finish the activation process.

Note: It is no longer possible to redeem a code using a Mojang account.

Here you can check how to activate the key on Minecraft.net: