Contact support and tell them how many you need and they'll try to accommodate you. There were a lot of people abusing the service and hosting hundreds of domains so now they're making everybody request them explicitly unfortunately. They've also had to suspend their .dedyn.io DDNS service indefinitely because of the abuse.
That's why we can't have nice things.
Please read up on DNSSEC because you will be required to turn it on for every domain you host with them.