How do I add a free product into customer's shopping cart directly

Maybe it has been done by many Zen pros already. But I have been struggling how to set this up.

Here is scenario. I want to offer my customer a free accessory item when they purchase one specific item. I know I can use either coupon or BetterTogether plug in to make it work. But I want to make it easier for my customer: When they add a product into their shopping cart, can zen-cart automatically add the FREE product into the shopping cart for them?

Thank you for your help

I can see how adding a separate product could cause some complications. For example, if the customer removes the parent product from their cart, the child product would need to be automatically removed as well.

The easy way is to make your free product into an attribute of the parent product.

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