WHMCS API is organized around Representation State Transfer (REST) and is designed around the use of resources. It expects standard HTTP verbs like GET, POST, PUT, and DELETE and returns HTTP response codes and JSON-encoded responses. To get started with the API, make sure that your system is running at least WHMCS 9.0.0-rc.1, and you have created product groups, products, and at least one user account.
Examples provided by this document use cURL, but you can make the API calls in any programming language.