To setup WHMCS plugin you need to enter your API username and password, which can be done in:
WHMCS Module Settings > CloudLunux Licenses.
- Please note, the access to the CloudLinux API is available to the reseller level accounts only.
The "username" here is your cln.cloudlinux.com login.
The "password" is the IP's registration token from the profile at https://cln.cloudlinux.com/clweb/account/profile.xhtml
CloudLinux WHMCS plugin documentation PDF file is attached to this article.