This article explains how to use Siri and Spotify together on iOS 12 devices with access to Siri Shortcuts. Once shortcuts have been created, they can also be used on Apple Watch Series 4 and 3.
How to Use Siri with Spotify on iPhone
Before you can use Siri with Spotify, you’ll need to install and create a Siri Shortcut specific to Spotify. Currently, Siri can’t interact with Spotify other than by opening the app on demand. The Siri Shortcuts app makes it easy to create a Spotify shortcut.
Download the Shortcuts app from the App Store.
On your iPhone's browser, tap the Spotify Siri download link.