This feature comes enabled by default when a domain is registered/transferred. If for some reason this feature was disabled in the past, it can be enabled by following these steps:
Log into the customer's portal.
Click on 'Domains' then on 'My Domains':
Find the domain you wish to not renew and click on the button just at the right side of the tool icon, then select 'Auto Renewal Status':
Click ‘Enable Auto Renew’:
Now the Auto-Renew Feature has been enabled!
IMPORTANT: Make sure you have a valid credit card or a recurring PayPal account on file.