Portal Overview, Terminology, and Configuration File

http://www.peakapi.com/uploads/PortalOverview.pdf

WHMCS Configuration

Login as the PeakAPI Portal Admin:

Edit WHMCS Configuration
https://www./whmcs/includes/api.php
Example: https://www.peakapi.com/whmcs/includes/api.php

UPDATE WHMCS IP CALL
To allow WHMCS to accept your remote API call, login to WHMCS:
Setup --> General Settings --> Other

The rest is really WHMCS tweaking to your specific needs.