Add discount -40% on working day to Free Weekend

You need to change your subscription (in Mijn NS, go to My products > Choose another subscription), then choose NS Flex Weekend Vrij again, but also select the option for discount outside peak hours:
 

If you've had this subscription for less than a month, you can't change it yourself and need to contact customer service by phone, chat, Twitter or Facebook to have them make this change for you.