If you want to use the WHMCS notification system, you have to create a new trigger. Please proceed as follows:

• Open the WHMCS notification configuration page.
• Create a new notification rule and select the API event.
• At conditions, select "Exact Match" and enter the value "custom.invoicenotifications.trigger"

The setup is now completed. Notifications will be sent via the WHMCS notification system if you selected this when setting up the notification.