Discount codes can't be zero. Need way for customer to enter custom code at checkout. Alternatives?

Hi! (Hopefully I am posting correctly and in the right place. I haven't posted before.)

My client wants to donate $1 for every item in the cart, BUT ONLY WHEN a customer indicates a certain code at checkout.

I thought this would be simple as setting up a discount code, but discount codes can't = 0. There must actually be a discount.

Does anyone have any ideas for an alternative way for a customer to note a code?

(I already tried adding a free add-on on the product page and it worked well but the client didn't like it.)