Hey there, your best option is to wait for the steam release as we won't be giving out keys for existing purchases. You wouldn't ask for an itch key on a steam purchase right?
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Actually it's pretty normal to expect games that I buy elsewhere which then become available on Steam, to be able to link the existing purchase there and own it on Steam as well. Some games are not done this way however, which is why I asked. That's fine though, I don't mind waiting to see it on Steam.