I'm assuming this is a voucher code for either entertainment or movies?
Usually what happens is there will even be a voucher code inside the Xbox packaging or game will send you a voucher code electronically. If you go to the my passes section of the my account area. You will be able to apply the code just use the apply voucher button next to the relevant pass type
If it's a code for an Xbox game, then that would be redeemed through Xbox live for which you would need a Microsoft account
Hope that helps
I do not work for Now . I am simply a Now customer trying to help
I am a Community Contributor
This means that I know a lot about the service. But just like you I am still a customer. This means I cannot help you with issues that would involve looking into your account directly. A member of the now TV forum team or live chat will need to assist you with these issues.