From here, you can select one of three co-op mode options: To activate co-op mode in Far Cry 6, open up any of the menu screens and click the co-op button in the top-right corner (the icon with the two silhouettes).
Once you complete the quest by building your first Supremo, you'll unlock the ability to play through the rest of the game in co-op mode. This mission appears very early on in the story, and acts as an introduction to Supremos in Far Cry 6. To play through Far Cry 6 in co-op multiplayer, you will first need to complete the story mission Du or Die.
How to play Far Cry 6 co-op multiplayer mode