To use preset nameservers for your new domain names while you register them with us, you have to first add the nameservers under 'Use Preset'.
To do this, please login to your account, click on any domain name, click on 'Modify Domain Setup' and under 'Nameservers' click on 'Use Preset' button and here enter the nameservers you wish to use.
Once the nameservers are added here, while you register a domain name, and once you have updated the WHOIS data, below the WHOIS form you will find the option to use preset nameservers, please click on it, and the nameservers will be added on your new domain name.