This special offer seems to apply to the first year only.
I set up a new ASUS account, and I used the code, but it didn't work and a message instructed me to contact the support desk.
I emailed the address given for the support desk and they responded quite quickly telling me that the code I had used (which was given at the link provided by @4wd) was intended for another account-holder. They advised me to login to my new account and I would be able to access the special offer from there.
I did not get around to doing that, but today received a reminder email to avail myself of the special offer, which was due to close in 24hrs. So I clicked on the special offer link given in the email reminder, and followed the steps. It worked without any hassle. There are options to pay via Credit Card or PayPal. There are about 17hrs of the special offer period left, before it closes.
So, get it whilst you can. Tempus fugit.