I've just done this too, it took me awhile to work out what to do!
You have to email email@example.com asking to register for purchasing permit vouchers online. Include your name and address and Council Tax Reference Number.
Then 3 or 4 days later they then send you a reply with an Applicant ID and Voucher Reference Number and a link to a specific page.
Once you've ordered you wait for another few days for them to be delivered.