I am not sure you can subscribe to that on realtime api. I would need to dig it deeper to be sure, but usually the user will not get a notification of that, unless he is part of the channel.
For monitoring this kind of event globally, you can leverage Rocket.Chat Apps, using the Event Interface, as described in:
okay
so what i did is ,i created a server and added that URL link
under Omnichannel->webhook and configured that to trigger this as on “chat start”
by this approach i am geting the roomID and visitor token