Adding CNAME record for a domain with subdomain
In this case your custom domain will look like
calendar.yourbusiness.com
Log in to your Cloudflare dashboard and select the site
Go to DNS -> Record
Click on Add record
Select CNAME as the Type
Add Name and Target
Disable Proxy status and enable Flatten
Click Save
For more information refer official documentation
Adding CNAME record for a root domain
In this case your custom domain will look like
yourbusiness.com
Follow the same steps mentioned above