Google is touting this as a limited time offer, applying to those that activate a new group plan or to those that add a member to an existing group plan and purchase two Pixel 3 or Pixel 3 XLs, or a combination of one of each from the Project Fi website. The activation must be full service, meaning that it cannot just be a data plan.
Google also has some other stipulations in place like "the devices must remain active for 30 consecutive days with 60 days of device shipment". After all of the criteria has been satisfied, the customer will receive an email notifying them that the credits will start to be applied to the account.
This isn't going to be one lump sum credit mind you, as Google will issue the credits in increments over some months. You can read through all of the terms and conditions to see whether this is something you would be interested in. If so, be sure to head to the source link below to take advantage of the promotion. Google states that this is a limited time offer and will only be available while supplies last.