I don't know if there might be an undocumented hack where you could modify the Add to Cart button code URL to pass the discount code to the cart and apply that automatically, but aside from that possibility, the only automatically-applied discounts are for cart-wide order-total discounts. Discount Codes applicable only to a particular product can normally only be applied by the buyer manually typing it into the cart screen as a "digital coupon" of sorts. Is there some reason why you couldn' just just change the item price in your Seller Admin and offer it as a "Normally $X.XX, now only $Y.YY" type of sale?