On their official blog they mentioned -
Here is 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. From there, enjoy access to your favorite Office apps and 1 TB of OneDrive cloud storage, valid for one year.
For complete terms and conditions you may visit here