This option determines how the default administrator's username for new servers is created from the domain name. The options are :

Full domain name
  The username is always the same as the full domain name, like `example.com`.
First part of domain
  The username is the first part of the domain name like `example` if available, otherwise the full domain name like `example.com`.
Same as prefix
  The username follows the same rule as the Domain name style in usernames config option above this one.
First part @ domain
  The username is always a full email address, like `example@example.com`
Regular expression custom pattern
  The prefix will be automatically generated based on custom regular expression pattern, like `u-[0-9]{4}` or `[a-z]-[0-9]{2}` or `user-[0-9]{2}-${year}-${month}-${day}` which will produce `u-1234` or `x-89` or `user-99-2021-12-25` accordingly.