Juli Clover, MacRumors:
When you go through the normal trade-in process on the trade-in site, Apple offers $150, but if you use the trade-in option during the checkout process of purchasing a new iPhone XR or XS, the bonus trade-in credit is displayed.
See the chart in the article for specifics. Nothing you need to do to get this bonus trade-in money, but if you are considering whether to sell your old phone or trade it in, this will help with the math.
This should be an incentive for me to trade in my four-year-old Six for a XR, but despite the fact that iOS updates will likely end for me within a year (?), I’m just not feeling the need.