To link your accounts, begin by logging your SHiFT account on the Gearbox website. Click on Setting, then go to the tab called "Connections". Click on the Twitch icon there, and authorize SHiFT to interact with Twitch. If you're not logged in to Twitch, now your chance to log in with the account you want to connect. When you authorize the connection, you are set to go.
The next time you watch a streamer play Borderlands 3, you have the chance to use the ECHOcast Twitch extension. This allows you to inspect the inventory, profile and skill tree of the streamer, and most importantly, allows you to claim chests!
How to claim chests by watching Borderlands 3 streamingAny time a streamer plays Borderlands 3 and finds a chest, you can click on a button on the ECHOcast extension to claim this chest. You only have two minutes to claim it after the streamer finds it, so you can't just get lots of chests by letting the stream run on another device.
After having claimed the chest, log in to your SHiFT account when you are in-game in Borderlands 3, and all the chests you have claimed will be available to open in Sanctuary III.