Hello,
We can see that this coupon can be applied automatically but using the percentage of discount.
Note that you can also set a "Flat amount" to your coupon.
Requirement: If a customer purchase 2 product from a specific category then he can get third product free from that product category.
With the solution we gave you, you'll just have to put 3 of these products on your cart, and then a coupon (with the price of your third product) will be auto-loaded in your cart, then you'll have a "BUY 2 GET 3nd FREE".
Also it will probably be best for your to use the last Hikashop version.