Domain Registrars
From WHMCS Documentation
We have prepared a video tutorial for this section @ http://www.whmcs.com/tutorials.php?i=7
Domain Registrars can be configured by going to Configuration > Domain Registrars in the WHMCS Admin Area. From here, you can set the default nameservers that WHMCS will use for domains registered by the system and if you have an account with one of the supported registrars, you should configure it here.
The following domain registrar companies are supported.