How can I access request in Play Guice Module?

I am writing an application which handles multiple systems. The user can choose the system which he wants to work with and I store that system ID in session (client session)

Now I have Service classes, lets say CustomerService.

class CustomerService(val systemID: String) {
    // Implementation
}

I want to use Guice to inject the Customer instance to the Controllers. But I want to instantiate the CustomerService with SystemID which is stored in the session.

How can I access request.session in Guice Module?

Edit:

Had simplified my code above. My actual code uses interfaces. How can I use assisted inject for this?

trait CustomerService(val systemID: String) {
    // Definition
}

object CustomerService{

  trait Factory {
    def apply(systemID: String) : CustomerService
  }

}

class DefaultCustomerService @Inject() (@Assisted systemID: String)
  extends CustomerService {
    // Definition
}

class CustomerController @Inject()(
                            val messagesApi: MessagesApi,
                            csFactory: CustomerService.Factory)
{
}

This gives me: CustomerService is an interface, not a concrete class. Unable to create AssistedInject factory.

And I do not want to put the Factory under DefaultCustomerService and use DefaultCustomerService.Factory in the controller. This is because for unit testing I will be using TestCustomerService stub and want Dependency Injection to inject TestCustomerService into the controller instead of DefaultCustomerService.