Tasks added with Siri will be added to your default list in Remember The Milk (for iOS 11+ and watchOS 4+). Learn how to change your default list.

You can also specify which list to add tasks to:

Add "Read Jane Goodall's book" to my Reading list in RTM

For earlier versions of iOS and watchOS, or if you are using the CalDAV integration:

Tasks added with Siri will be added to your default list in Remember The Milk. Learn how to change your default list.