Juli Clover, reporting for MacRumors on the the third beta of tvOS 9.2 seeded to developers earlier today:
There’s now support for onscreen text entry via dictation in countries where Siri is available. When updating to tvOS 9.2 beta 3, users will be prompted to enable or disable dictation. With dictation, Apple TV users can dictate text and spell user names and passwords rather than typing them. With dictation enabled, the tvOS search bar alternates between a blank search field and an option to hold the Siri button to dictate text.
Feels like another feature that should have been there since tvOS debuted. Maybe the ability to actually link to tvOS apps will be next.