Step 1] Install WHMCS on your domain.
Step 2] Login to WHMCS admin panel.
Step 3] Go to Setup→General Settings→Security→API IP Access restriction
(Domain IP is to be added here)
Step 4] Log-in to the manage portal.
Domain: Domain for which WHMCS single login settings are to be done is to be selected from the drop down.
Billing URL: WHMCS URL is to be added for the Billing url.
Admin username: The admin username for WHMCS is to be specified here.
Admin password: The admin password for WHMCS is to be specified here.
API key: The API key for WHMCS is to be mentioned here.
Status: The status can be either enabled or disabled from here.