AFAIK there is no discount code, you need to enter using the link from the training page:
and it should point you to an exam code with an F (free) at the end:
Those are the exams I enrolled...
Ups, I have just tried to follow the link on the exam and takes me to this page rather than to the Pearson Vue one:
This changed in the las few days....
All slots have been booked for now. That might be the reason
All slots are full. So Oracle has removed the -F exams from Pearson. Test takers have to register in the above form and Oracle will get back for the free exam.