When a request/record is created on the platform, the domain would be determined by the current login session domain.
If a User "ABC" is creating the service request, the system takes the domain that this user has selected on the domain picker.
Even though "ABC " user is creating the request for another user "DEF", the domain of the session where the record is created would be set for that requested record.
As per the business need, if we would like to have the requested for user's domain as request domain, then it can be achieved in 2 ways.
1) ITIL user can change the domain to "Request For" user's domain in the domain picker, before creating a request.
2) The same can be achieved using a business rule to change the domain to the "Request For" user's domain "onSubmit" of the request.