It's members only.
Xbox One Preview Members: Halo Wars will be playable in PREVIEW via Back Compat later today (under Ready To Install) pic.twitter.com/Dm8wTWkJGa
— Larry Hryb (@majornelson) March 17, 2016
Major Nelson tweeted out the announcement earlier today.
If you're an Xbox One Preview member, you'll be able to give Halo Wars a whirl later today thanks to the backwards compatibility feature. You can find it under the 'ready to install' menu.
A few more backwards compatible tiles were released for Xbox One yesterday, including Alan Wake, and Castlevania: Symphony of the Night.
Other upcoming games for this feature include BioShock, BioShock 2, and BioShock Infinite - although if rumours are to be believed, we can expect to see the BioShock Collection for Xbox One, PS4, and PC soon.