Customisation and Advanced Features
From WHMCS Documentation

Setup/Configuration
General Configuration | Payment Configuration | Products/Services Configuration
Support Configuration | System Configuration
General Configuration | Payment Configuration | Products/Services Configuration
Support Configuration | System Configuration
Build for flexibility and customisation, WHMCS is a powerful too that you can make your own. Below are some helpful links to help you customise and power up your WHMCS.
- Creating a Custom System Theme
 - Customising Email Messages
 - Customising Text and Phrases
 - Creating your own integrations
 - Connect to WHMCS Externally via the API
 - Change Navigation Bar Links
 - Change Sidebar Links
 - Use Data Feeds to display product information on your website
 - Activate Social Media Sign-in
 - Set up automated email marketing campaigns via the Email Marketer Utility or Mailchimp
 - Use Google Analytics to track visitors and conversions
 - Configure external apps to access WHMCS via OAuth