Cheapest I've seen it, £44.99 with £2 off if you have prime.
Although this is the physical PC edition, it simply has a code in the box for the play anywhere version (as well as installation disc's for the PC version).
You just redeem the code as you would any other Xbox Live code.
On amazon page:
This is a Digital version of Halo Wars 2 which you can install via physical media on Windows 10 (64 bit). This Digital version of Halo Wars 2 is an Xbox Play Anywhere game. During installation, you will be directed to redeem a code for Halo Wars 2 on the Windows Marketplace using your Microsoft Account. From there, Halo Wars 2 will be available to install on any Xbox One or Windows 10 device that you sign in to using that same Microsoft Account.