I assume you are talking in terms of a report of an accident under ESIC Rules. However, if it is a factory, I think the factory (principal employer) needs to send an accident report to the factory inspector, independent of what is going to the ESIC. That is because the contract worker is a worker in the factory, and the occupier of the factory is responsible for ensuring safety and a proper working environment.
Since the contractor has his own ESI registration, the employee who met with the accident will be under that registration number only. He will not be available in the Principal Employer's employee list when he logs in for submitting the accident report. Therefore, the contractor only can send the accident report. You have to furnish the details to the contractor like the exact place of the accident, what the employee was doing at the time of the accident, nature of the injury, whether fracture, cut, contusion, etc., and location (right hand/leg, eye, chest...) and so on so that he can submit the Form 12 online as well offline (submit the hard copy to the Branch office and Dispensary).
Regards,
Madhu.T.K