So here is an example:
Buy 1 Get 1 Free:
Add 2 x MK-677 then apply coupon
Buy 2 Get 2 Free:
Add 4 x MK-677 then apply coupon
Buy 3 Get 3 Free:
Add 6 x MK-677 then apply coupon
Once you apply coupon you will see the discount applied above the Grand Total.
Make sure you enter the total number of items you are expecting in your cart including the BOGO freebie.