An administrator configures a new destination list in Cisco Umbrella so that the organization can...

To block all subdomains of domain.com, the administrator should configure the domain.com address in the block list. This is because Umbrella automatically applies a left side and right side wildcard to every domain in a block or allow destination list. Therefore, adding domain.com to a block list will result in requests to domain.com or its subdomains, such as www.domain.com, being blocked. Adding a wildcard character (*) is not supported and will not work. Adding the *.com address in the block list will block all domains that end with .com, which is not the desired outcome. References:

Understanding Destination lists supported entries and error messages

Wildcards and Destination Lists