Adding CNAME record for a domain with subdomain
In this case your custom domain will look like
calendar.yourbusiness.com
Log in to your Hostinger dashboard
Go to Domains -> Domain portfolio
Click on Manage next to the domain
Go to DNS/Nameservers -> DNS record
Select CNAME as the Type
Add Name and Target
Click on Add record
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. Hostinger automatically converts CNAME to ALIAS for root domains