3. Then, you need to put the ‘nowpayments’ folder into the modules/gateways directory.
In the WHMCS the installation process follows 4 simple steps:
Log into your WHMCS admin panel and click the button that says “Setup, Payments, Payment Gateways”;
Then, in the “Activate Module” dropdown, select “NOWPayments” and click the Activate button;
After that, in the NOWPayments configuration form and enter your API Key and your IPN Secret (from My Account -> Edit Settings.);
To finish up the process, click 'Save Changes' and you are good to go!