You will need the Lumia Offers app installed on your 950/XL and once you follow the steps in the app it will send am email with a link and promotion code.
If you don't have the Lumia Offers app installed, you can install it from link below:
https://www.microsoft.com/en-au/store/apps/lumia-offers/9nblggh2kbnv?tduid=$290e85a47d2191b1b1a8d7d068d598b1$$263915$$2775081$$$$$