https://www.gamespot.com/articles/get-your-free-overwatch-golden-loot-box-for-twitch/1100-6461576/

Who doesn't like Overwatch loot? Every player could use some more skins, emotes, and voice lines for their beloved heroes. If you're a Twitch / Amazon Prime member, you can get a free Golden Loot Box right now. All you have to do is hop over to Twitch and click the crown-shaped icon near the search bar. There you'll find a new Overwatch bonus waiting for you. (And if you haven't grabbed September's free PC games, you can pick them up as well).

PC players can hop right into Overwatch to find their loot box waiting for them. Console players can click here to link their PlayStation or Xbox Live account with their Blizzard account. Once you've jumped through the hoops, you're good to go. Read the FAQ if you run into trouble during any of the steps.