How to play Coop Mode?

[CallMeMax]

I have seen, that in-game there is the option for coop play. I would like to know how to join another player? I have 2 computer in a local area network which can ping each other using their local ip addresses. If I bin to my local IP address instead of 127.0.0.1 using the "ip" field in assets\database\configs\server.json, I can reach the web service from both computers. But how do I run the game on the other computer? If I click on "Start Tarkov" the promt only appears on the computer where the server runs.