Tunnels are mapped as ordinary segments, with the attribute Tunnel checked.
Use as Elevation a negative value, like -1.
The complication with navigation through tunnels is that generally they obscure GPS communication and cell phone data connections. The Waze servers track each mobile device (driver) as they enter and exit each segment along their route. With the start and end times of each segment, the Waze server can calculate the average speed of drivers through that segment. This is how the Waze server can determine if the tunnel traffic is flowing normally, or if there is a slowdown of the vehicles in the tunnel even without the GPS tracking information while inside the tunnel.
The name of a Tunnel can also be put in the street name if the tunnel is important for orientation and is regularly used in communication about, for example, road closures.