If you're signed up for Twitch Prime—which is included in an Amazon Prime subscription—all you have to do is go to this page to claim your goodies. You'll be wanting one of his high-tier skins: I'm a big fan of the wooden one, which is third from bottom on this list of all his outfits.
Prime members can also expect free golden loot boxes, which guarantee you at least one legendary skin each, over the coming months. They'll contain loot for all heroes, not just Wrecking Ball. You can claim one in September, and one in October.
Subscribers got a whopping 21 free games this month, some of which are still available, and can expect Jotun, Death Squared and Antihero for free next month. Due to a recent price increase, Amazon Prime costs a fairly steep $119 a year. If you're interested, go here.