You cannot redeem the Anthology code on Steam.
It's supposed to be entered here: https://www.larianvault.com/voucher.php
Create an account and the code will be accepted.

Besides, I don't think it has high priotiry at the moment, backer bonus still isn't working, at least for me.

I might just refrain from playing for a couple of weeks until it's all sorted out and hopefully more AI personalities are added.