Adding CNAME record for a domain with subdomain
In this case your custom domain will look like
calendar.yourbusiness.com
Log in to your DigitalOcean dashboard and Go to Networkings -> Domains
Select your domain form the list
Click on CNAME section
Add HOSTNAME and WILL REDIRECT TO
Click on Create Record
For more information refer official documentation
Adding CNAME record for a root domain
We cannot set a CNAME record for the root domain in DigitalOcean