How do you add a new Photonnetwork room variable?

RAKUS

For my game, i want to check if a match has begun in the current room, which the client is in.

For that I want a custom variable that applies to every lobby. For example a boolean called matchStarted.

Then ideally I want to check if a match has started with something like this:

if (PhotonNetwork.CurrentRoom.matchStarted)

How do I go about this?

Thanks in advance.