Make sure the domain exists on the primary name server, backup primary name server, and 2nd backup primary name servers prior to creating secondary domain.
In addition you need to have whitelisted our AXFR IP's for the zone addition to be successful.
For more information on how to whitelist our name servers see What IPs need to be whitelisted for zone transfer and NOTIFY to Neustar?
1. Log into portal.ultradns.com
2. Click on Domains tab
3. Click on + Add Domain button
4. Enter Domain Name:
5. Click on Secondary Domain
6. Enter Primary Name Server: (Required)
7. Enter 1st Backup Primary NS: (Optional)
8. Enter 2nd Backup Primary NS: (Optional)
9. Enter Notification Email: (Optional)
10. Enter Change Comment: (Optional)
11. Click Save button