1. Create a discount in /admin/discounts_next/new of the amount of $ of the free giveaway. You could use 100% discount, but I prefer a $ discount. Let's say your giveaway product is $5.
2. Add a Minimum purchase requirement. Here, for example, I add $20 (which is the cost of the Product A, the one I need to have in my cart to get the giveaway for free).
3. Then, select the product you want to give away for free. Here:
4. I would also probably limit to one per customer
5. Make sure your product A (the one I need to have in my cart to get the giveaway for free), has a unique Product Type, in the product page of the Product A. That means this Product A has a product type that no other product has. Example here:
6. Inside our app, create the discount rule. Make sure you click on Specify product type and/or vendor to see the Product type restriction. Here's an example: