Starting on May 21st, players will get a free Twitch crate every time they make an ESO purchase through the Twitch store. These crates have a variety of consumables and a chance at a very rare mount and pet.
If you want a crate without paying money, you’ll have your opportunity by watching supported Twitch streams (coming soon). View all the way to the end of these special streams, and poof, you’ll get a free crate. Cool, eh?
Source: Elder Scrolls Online. Thanks Jason!