I noticed these instructions were shared by Rise of Industry previously. Let me know if these steps help:
Step 1 - Find or re-request the download page from your original Itch.io purchase email
If you haven't done so, find your original email with the purchase link.
Can't find your email? Go to this page: https://itch.io/docs/buying/already-bought and you can request that the download links be sent to you!
The page should look something like this, so click the "download page" link:
Step 2 - Click "Get Steam Key" and follow the instructions
Click the "Get Steam Key" button shown (check image). 
Follow the instructions to link your Steam account, verify your email if necessary and then copy the key to your clipboard.
Step 3 - Add your key to Steam
If you'd like, you can link your Steam key early, so on the 9th February all you have to do is install and run it.
Click on "Add a game..." on the bottom left of the Steam Library, then on "Activate":
Enter the key, as shown:
If everything goes well, you should get it confirmed:
Step 4 -
You can now download, install and play the game from your Steam library!
Troubleshooting
Q: I didn't create an itch.io account when I bought the game!
A: Go to the bottom of this page: https://itch.io/docs/buying/already-bought
Q: When I open the link to get my Steam key itch displays a 404 error.
A: This will happen if you open the link in a browser that is not logged in to your itch account. Ensure you're logged in to itch and try again
Once again, thank you for joining us on this wild ride!
https://discord.meeplestation.com Here is a discord link where you can speak with us!