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.