1 - The Xbox Game Pass Ultimate code is expected to be provided during the setup of the Backbone. When you connect your Backbone One to your iPhone and open the Backbone app, the setup process is expected to automatically start.
2 - You can also check within the Backbone app with your Backbone connected and clicking on the tile shown below. Here you should be able to get your code.
NOTE: If you see a "Check the Instructions that came with your code. You might need to redeem it on a different web page" error, this may mean you do not meet the eligibility requirements to redeem the code. If you believe you meet the requirements, but are still receiving an error message please visit Xbox support for more information.