If you are a registered Apple developer, you can register here to apply for a new Apple TV developer kit. This means you can test the new Apple TV (and your apps on the new hardware) ahead of general release. Availability is limited and Apple will prioritize developers that already have apps on the store.
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You can register for a developer kit if you have been registered in the Apple Developer program as of September 9, 2015 at 9:00 a.m and are registered in one of a select number of countries, including the United States and United Kingdom.
The Apple TV Developer Kit includes the new Apple TV, Apple TV Remote, Power cord, Lightning to USB Cable, USB-A to USB-C Cable, and documentation.