Users of the Xbox One X and Xbox One X will get FreeSync 2 with HDR support, and even Xbox One users will get support for first-generation FreeSync. The feature will be rolled out to Alpha Ring for Xbox Insiders users within a couple of days. It's unknown when final support will be ready.
FreeSync 2 with HDR for Xbox One S/X should be available in the Alpha Ring for Xbox Insiders within the next few days. Original FreeSync will be available for the standard Xbox One. You'll need a FreeSync-compatible monitor.