The Stadia Premiere Edition is a hardware bundle from Google intended to facilitate this OEM's own in-house game-streaming platform. To recap, it consists of a 4K-capable Chromecast Ultra and a first-party controller. It is currently priced at US$99; however, some new OnePlus customers might get it for free instead.
The company is now offering these buyers a code to claim one of those bundles, issued on the purchase of some new devices from the company. They include the 9 Pro and its vanilla 9 slbling; its predecessor the 8 Pro; its refresh the 8T, or the inaugural Nord.
Those who buy any of these units from oneplus.com have 15 days to avail of their free Stadia Premiere Edition. They can then presumably sign up for Stadia (presumign they find that necessary) to play with the relative advantages of OnePlus' Fluid AMOLED and Warp Charging technology.