I bought an Xbox One S during black Friday from microsofts uk store and just logged into my rewards account to see I have been credited 50000 reward points which equates to £30. If your going to buy an xbox one from the microsoft store make sures its using the same email linked to your gamertag and for xbox live rewards. By doing this as long as its by the 31st December you should get £30 to spend on the Xbox marketplace. If you have already bought one and not signed up perhaps live chat can sort something out.
I hope this helps someone I didn't even know this offer existed until I logged into my account.