From Slickdeals: LavatoryMaster1
The Monster Energy Promotion is offering a code for a free digital copy of Halo 5: Guardians just for signing up for an account.
1. Go to: https://earnrewards.com/monsterhalo
2. Create an account (or log into existing account)
3. On the page where you log a receipt to get the double XP tokens in Halo Infinite, scroll to the bottom.
4. You should see details regarding a free digital copy of Halo 5 Guardians (codes are limited)
5. Follow the short steps and you should be presented a code to redeem at redeem.microsoft.com
The details imply you do not need to have logged any receipts towards the double XP codes for Halo Infinite, but I had already submitted a few before noticing this promotion.