Adding CNAME record for a domain with subdomain
In this case your custom domain will look like
calendar.yourbusiness.com
Log in to your Porkbun dashboard
Go to Account -> Domain Management
Click on DNS next to your domain
Select CNAME as the Type
Add Host and Answer
Click on Add
For more information refer official documentation
Adding ALIAS record for a root domain
In this case your custom domain will look like
yourbusiness.com
Follow the same steps mentioned above except selecting the record type
Select ALIAS as the Type