/UserDomain:"domain_name" Specifies the domain name for the user name (account credentials) used for the operation. It also ...

/UserDomain:"domain_name"
Specifies the domain name for the user name (account credentials) used for the operation. It also helps to specify the forest where you plan to install the domain controller or create an RODC account. If no value is specified, the domain of the computer will be used.