itch.io

In the discord message about the steam version, he said something like " if you paid, you´ll get a key"

EDIT: If you previously purchased the game on itch.io, you will soon be recieving a Steam key for the game. This will take about a day (perhaps more) to set up.

that were the original words

Go back to the download page from Itch.io and there is a link to get the steam key there.