https://store.steampowered.com/app/2079360/EXIT_2__Supporter_Pack/
First of all I have to say that EXIT 2 & the Multiplayer will stay Free to Play.
However, if there are people who would like to voluntarily support the game, they can do so with the Supporter DLC.
EXIT 2 | Supporter Pack
Source: Steam Community (opens in a new tab)