in your products table, you would need to add a new column to hold the coupon code for the product that should be added.
then create a recordset and set the filtering options to filter the coupon code column on the coupon code form element.
go to the server behaviors panel and click the plus button and select eCart -> add to cart -> add to cart.
set the trigger to Recordset not empty, then on the bindings panel, bind the cart columns to the corresponding recordset columns.