On Amazon UK then:
It's not clear how this is policed, so it's worth trying your luck with the Lumia Offers application even if you bought your 950 or 950 XL already. Let us know how you get on!
How to claim the offer
Step 1. Open the pre-installed Lumia Offers app on your Lumia 950 or 950 XL. If you don’t have the app installed, download it from Windows Store, sign in, and accept the terms and conditions.
Step 2. Choose 'Display dock' from the 'available offers' list
Step 3. Enter your email address and tap ‘continue’ to get the email with a unique promotional link.
Step 4. Click that link and follow the instructions to order your Display Dock from your local online Microsoft Store. See the full Terms and Conditions for more details