Limited support
This feature is only present in Portal 2: Community Edition and Portal: Revolution.

mm_dedicated_force_servers

🟪 Executable on Client and Server

mm_dedicated_force_servers <value>

Comma delimited list of ip:port of servers used to search for dedicated servers instead of searching for public servers. Use syntax publicip1:port|privateip1:port,publicip2:port|privateip2:port if your server is behind NAT. If the server is behind NAT, you can specify 0.0.0.0|privateip:port and if server port is in the list of mm_server_search_lan_ports its public address should be automatically detected.