As stated in the promotion itself, your bonus V-Bucks code will arrive via the Xbox message center within 2 weeks of purchase.
If after 2 weeks you have not received the V-Bucks in your XBL messages. We have received further information in regards to this promotion that all V-bucks will be received no later than 12/31/18 to those eligible for the promotion.
Also as well here are the terms and conditions for the offer. (Check for Eligibility)
Terms and conditions:
Valid only for new subscribers or users who canceled their subscription before 10/1/2018. Limit: 1 offer per account; credit card required. Only valid through link in this offer, in all Xbox Game Pass markets excluding Quebec. After first month, subscription
continues to be charged at the then-current regular monthly price (subject to change), unless cancelled. Plus applicable taxes. Game catalog varies over time. Not valid for existing subscribers . After promotional period, subscription continues to be charged
at the then-current regular monthly price (subject to change), unless cancelled. Plus applicable taxes. Limit: 1 offer per account. Only valid through link in this offer, in all Xbox Live markets excluding Quebec. Credit card required.