You need to register nameservers under Domain Host Summary section.
After login your account, go to Domain Manager, select domain you want to create private nameservers.
On domain management page, scroll down to the bottom you will see Host Summary section.
Add nameserver record in Host Summary section
ns1.your-domain.com -> dns-server-ip1
ns2.your-domain.com -> dns-server-ip2
You also need to add A record for ns1 & ns2 in dns zone of your domain.
Your nameservers will be ready to use after 12-48 hours which is the time required for dns propagation on the internet.
