We still don’t really know much about Fallout 76, but if you figure you’ll buy it no matter what, you can preorder now on Amazon (PS4 | Xbox One | PC), with 20% off at checkout for Prime members. As always, you won’t be charged until it ships, and you can always cancel if they do, in fact, turn it into an MMO.
Update: Kotaku has more details on what to expect from the game.