If you've launched the game, Steam should already have made an account for you that you'll have to unlink.
Step 1. Go to the log-in page on the Haven & Hearth website. Then, click as indicated to make your unnamed steam account into a Haven account.

- NVIDIA_Overlay_sHGJwCnp3J.png (148.78 KiB) Viewed 10652 times
Step 2. Follow the steps of the earlier section, except
unlink the Steam account on this new account instead.
Step 3. Log into your old account, then follow the steps on that prior section once again to link your Steam account to your old account.
WARNING. As of this guide's date of writing, you can't transfer your characters out of that Steam account you made by accident, so, if your old account was verified or subscribed,
DO THIS AS SOON AS POSSIBLE so you don't lose progress. As an aside, this last thing worries me. Lots and lots of people are going to play a bit now that the game's released and are going to have their characters stuck on an unverified account without much recourse. Is there an alternative I haven't thought of?