I just purchased the $399 Skywards Plus subscription and used it to purchase a roundtrip itinerary for 4 passengers; I was able to use the 20% discount/voucher to save 160K points. The process was very glitchy/quirky, and Emirates was unable to provide much help via phone or chat support. Here's what I figured out; I hope this helps someone:

• Click through the "20% Discount Class Reward" card to apply to promo code as described in Post #45.
• The system will only apply the discount to an adult fair. So, I had to book my party as 4 adults, even though it's actually 2 adults and 2 kids.
• I was booking Business Class. However, unless you have enough miles to purchase the itinerary without the 20% discount, you must enter Economy Class in the search engine. Otherwise, when you attempt to make the purchase, the system will error out and say that you don't have enough miles to purchase the flight. Once the flight options pop-up, you can then select Business Class and everything will work as it should.