You can redeem it on Xbox.com with your regular account, you should probably do it on your Xbox One with the region set to US though. There might be error messages and shit otherwise if the region on Xbox.com is wrong or the system hasn't created your us billing info yet or whatever.
Just change the locale, no need for extra accounts or shit like that.
thanks. Do I need to change any address? How will the xbox marketplace know not to charge me sales tax?
Edit : redeemed Xbox code ok, now it's asking me for billing info. Assume I put in the Oregon address in the OP to avoid tax, but how about the phone number?
Edit: all done, thanks for the info. Grabbed Zoo Tycoon, Powerstar Golf and PvZ garden warfare for £60 - would have been more like £110 bought in the UK. Just need to remember to switch my amazon billing and delivery address back to the uk before I order anything else
Can I change my locale back to the UK immediately, or should I wait until the games have finished downloading and installing?