The "Provider" screen.

Provider information on the service detail level is required when requesting a service provider, specialist, or specialty entity for this service that is different from the provider, specialist, or specialty entity identified in Loop 2010EA (Patient Event Provider). If Loop 2010EA is not valued, Loop 2010F must be valued for each service associated with this patient event. If this information is not required by the implementation guide, it may be provided at the sender’s discretion but cannot be required by the receiver.

While the 5010 standard allows up to 10 Service Detail Level providers, the HIPAA Authorizer limits it up to three providers that can be added. Note that you have to click "Save Provider Info" to have the information actually added to the transaction.