Want to create an order form that will add an extra bottle of product when the customer purchase one bottle


would love to do buy one get one product on the order form.
So basically when the customer purchases via an order form, want to modify the order to either add extra product/lineitem that way fulfillment knows to add 2 bottles instead of one bottle or sometimes add another product for free.

How can this be achieved when selling its via infusionsoft order from.
I am thinking Purchase Created as Goal and then use legacy action builder to throw in another bottle will that work?

Thansk in advance