Does anyone have experience with this? I've tried logging into the website, but there's nowhere to enter codes. If I try to click on Redeem in game, it shows me past codes I've entered (none as of the transfer) but does not give me a spot to enter new codes. How do I enter these?

Is this what you are looking for?  

https://payment.naeu.playblackdesert.com/en-US/Shop/Coupon

Thanks

Yes, steam players can now redeem web coupons on site.