Is the "Download Key" for steam? If so, it doesnt work. If not, then what exactly is it for?
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Your itch key should look like that: https://ludenio.itch.io/warnament/download/SP0-es****eFcqTBKh
It's a link. Just open it in your browser and after that, you can download the game using your itch account from this page: https://ludenio.itch.io/warnament/. You should find a green button with the text "Download" on it.
Here is the help page to with a form to receive a download link to your email https://itch.io/docs/buying/already-bought