Yes, the panel specialist/hospital can apply an outpatient GL on behalf of the employee (if the company settings allow so) only on the actual appointment date.
However, this method is not recommended as there may be waiting time which may disrupt your actual scheduled appointment time.
Hence, it is recommended to pre-apply your outpatient GL using the HealthMetrics mobile app. Once that's done proceed only to the hospital after your GL has been approved.
For more info on how to apply outpatient GL from your HealthMetrics mobile app refer here: Applying/Requesting Guarantee Letter