Solve the problem of unavailable multiplayer server

If you bought Watch Dogs directly and only on steam
You won't have access to in-game multiplayer!

Why?
Because steam's “migrated” key is only “semi-approved” by ubisoft and doesn't grant you access to their multiplayer server (it's blocked somewhere anyway).

The solution:
Buy the game directly from the official ubisoft store,
You will then have two different versions of the same game in your library
( the two game keys will be identical )

Install the ubisoft store version you've just bought.
Launch and play for about 10 minutes, until you reach your hideout for the first time to save (to synchronize everything).

Once you've done that, quit the game and relaunch the steam version (ideally from steam).
You should now have access to multiplayer.
Request a refund for your freshly purchased ubisoft store version.
( This procedure/problem is probably the same for Watch Dogs 2 and Watch Dogs : Legion )

If you had already played the game on your steam version, don't worry, the saves are separate for both versions )

( It may be (?) possible to have this corrected by contacting support, it'll probably take longer, but we'll have to check)