Make Discount Code Apply Button More Obvious

The Webinar Registration page was recently changed so that after entering a discount code, the registrant would click on a reasonably obvious "Apply" button. It might even have said "Apply Discount".
It is now a fairly subtle "+" button in the discount code field.
Within a few days of publicizing a webinar that linked to the new page we had two customers entering the discount code (which would have allowed them to register for $0), decide that the page was expecting their credit card information to proceed (as the register button was not enabled), and click on the "Pay $150 to Register" button assuming that the discount code would change the amount to $0 and they wouldn't have to pay. Not a clever move, I agree, but that's what they figured they needed to do.

We refunded their money, but that costs us money as Stripe doesn't refund us their fee.

The "+" button is too subtle so invites this sort of error.

Please make it something much more obvious. 

The web page already describes all the fields in English and uses text on the "Pay 150 USD and Register" button, so why not keep the button using English "Apply Discount Code", enabling it when text is entered in the field?

Thanks.