Login to your resellers account.
Select Account Settings > global settings.
In Name Server Mask enter your-domain-name.com
Then open a support ticket (email to firstname.lastname@example.org) with your reseller account number and the name servers you will be using, and we will update helm so all your clients see your name servers as default.
WE CAN ONLY PROVIDE NAME SERVERS TO A DOMAIN THAT IS HOSTED WITH US AND REGISTERED/TRANSFERRED WITH US !