Network ID Provider
The Custom Network ID option requires a logic to return the Network ID to associate with the player.
In the image above, the "NetworkIdProvider" is the object with a component NetworkIdProvider which has a method called "GetNetworkId".
To create a NetworkIdProvider provider you will need to create a script to provide a custom ID, this script shall implement the interface IInformationProvider to return the ID to be associated with the player prefab to be spawned ( see Providers Scripts ).
You can also check the example script NetworkIdProvider in the examples folder. This script has a full implementation of how to provide custom IDs.