"The API is an interface to allow you to access and perform actions inside WHMCS from external applications and scripts. It allows you to furthur automate and integrate your WHMCS system with other processes you have.
The API is a work in process and is being continually developed and added to as more features are requested. If you have a need for a feature that is not currently available, feel free to contact us and let us know and we'll see what we can do." WHMCS.com
The only thing you need to do is create a product/service in the admin panel of WHMCS. Once that is completed just go to yourwebsite.com/cart.php and you'll see your product/service.
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