Download "zerotier" from zerotier(dot)com /its like Hamachi but better imo/

Install the app!
-After the installation you 'll find the program at the buttom of your taskbar.
-The app doesn't have UI (user interface) no worries about this(;

Go back to zerotiers website and register

-After you login click on "create network"
⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀
-Click on the created network

-Copy the NETWORK ID

Right click to the zerotier app at the taskbar
Select join network and past the NETWORK ID

You must give the NETWORK ID to your friend to join the game.
-Your friend must follow Chapter 4

!After this you must go back to zerotiers website open your network, scroll down and enable auth for all joined networks!

If its ready, open the game, choose a server (eu 1 or 2) select a game lobby.

If you join the lobby you can start the actual "mission" lobby and your friend can join ^^.

I hope this guide helps!

If ur game crashes in mp u should try this:
https://steamproxy.net/app/1930/discussions/0/451851477871359094/

(rate me 1 to 10 xd)