[Multiplayer] Should I use NetworkManager.Singleton or [SerializeField] a NetworkManager gameObj with NetworkManager component in lobby?

In regards to the lobby for matchmaking,

1. Should I use NetworkManager.Singleton.StartMatchmaking()?

2. OR [SerializeField] a NetworkManager gameObj with NetworkManager component and use myNetMan.StartMatchmaking()?

3. Or should I use myNetMan.Singleton.StartMatchmaking (I think that existed)?

I saw several examples throughout my research, and trying to find the "standard". So far I'm thinking about #2?