https://wccftech.com/fallout-76-steam-overlay/

Fallout 76 hasn’t been released on Steam, and it seems like it won’t anytime soon due to a variety of reasons. If you really cannot play any game without the Steam Overlay, you will be happy to know that it’s now possible to do so in a convenient way thanks to a new mod.

The Fallout 76 With Steam Overlay mod enables the overlay automatically when launching Fallout 76.