Follow the below given steps to setup Crons within WHMCS.
Step::1 Login to your WHMCS.
Step::2 Hover over the Setup section and click on Automation settings.
Step::3 Copy the Cron command and then proceed to the next step.
Step::4 Login to cPanel interface.
Step::5 Go to Cron Job section.
Step::6 Now paste the command in the command box and click on Add New Cron Job.
That's It !