MD-11 First Officer reported during engine start at the gate the aircraft began to roll so he advised the Captain who activated the brakes.

Narrative

During pre-flight the Captain left the parking brake off; which is normal for most operations. However; this was a [special] flight which is non-standard. Shortly after starting 3rd engine the aircraft began to roll. I noticed the movement and called it out to the Captain and we immediately came on the brakes. In the majority of operations; the parking brake is not on for engine start. There is also standardized communication with the ground in which parking brake status is confirmed between cockpit and ground personnel. This is the normal habit pattern. [These types of] operations are different and there is even a note in the guide mentioning leaving the parking brake on. However; neither the Captain or I have done many of them; and reverted to normal habit pattern.If ground personnel are [company] employees; they should be using standard terminology before engine start and verifying parking brake status. If not; this may not be an option; but locations where we typically do [these type of flights] could be advised of this needed communication between ground and cockpit. I know it is written in the ZZZ guide; but if this is a recurring issue; maybe include it in the email in big red letters.