We can't actually remember a time when Amazon Prime Gaming has given away an Xbox title as part of its collection of freebies, but that's what's happening this month, with Fallout 76 being the game that's getting the treatment.
Specifically, Amazon Prime members will be able to claim Fallout 76 for "free" on April 11th, in the form of an Xbox code for Xbox consoles and a Microsoft Store code for PC. Here's the confirmation of this:
April 11 Fallout 76 [PC via Microsoft Store Code; Xbox via Xbox Code] — Venture forth from your bunker 25 years after nuclear catastrophe. Explore a vast wasteland, devastated by war, in this open-world multiplayer addition to the Fallout story.
This is a permanent copy of the game, so it won't expire at any point, meaning you may as well claim it if you can. Don't forget that Amazon Prime offers free trials in some cases as well, so it's possible that you'll get this entirely for free.
When April 11th rolls around, all you need to do is head to the Prime Gaming website, log into your Amazon account with your details, and then follow the instructions to claim your Xbox copy. A very nice bonus indeed!