To give a customer a free gift(s) in their first box, we have to manually check each order to see if a coupon at £0.00 has been redeemed, when it is, we add the freebie to the "Shipping Items -> Select shipping items"

Instead, we'd like the coupon be attatched to a SKU, so when the coupon is redeemed, it automatically adds the SKU to the order, for free. I know this is possible with surveys & possibly funnels, but this route means every customer is entitled to it. It needs to be specifically when a code is entered so we can track where they've come from such as FB ads, Social posts, email marketing, YouTube unboxings. Each platform has a unique code.