Flutter - How to use Navigator.replaceRouteBelow()?

Question

What is the anchor route parameter that you need to pass to Navigator.replaceRouteBelow()?

I know it needs to be a route, but since the Navigator API doesn't expose the routes it's managing, how do you reference the current route, so that you can replace the screen below it?

The scenario in which I'm trying to use it is to replace the screen below the current one, so that Navigator.pop() takes the user to a different screen than the one that initiated the push.