According to Microsoft, “Here’s how it works – Open the Lumia Offers app on your qualifying Lumia 950 or Lumia 950 XL or download it from the Windows Store. Choose ‘1-year Office 365’ from the ‘available offers’ list. Enter your email and tap ‘continue’ for instructions to claim your subscription.”
However as Microsoft points out, not all Lumia 950 or 950 XL owners will be eligible for the promotion, simply because this offer is not available in every market both handsets are in. According to Microsoft, this only seems to apply to countries such as the U.S., UK, Germany, France, Denmark, Finland, Norway and Sweden. So if you don’t live in any of these countries we guess you’d be out of luck.
In addition to the 1-year subscription, users will also be given access to 1TB of OneDrive cloud storage that will also be valid for a year, so if you already own either device, then you should probably go ahead and claim the offer while it lasts.