You can now ask Siri to play any song on Spotify

Reddit:

I am running both iOS 13 beta (it gets publicly released in a few hours) and Spotify beta. iOS 13 is required, as well as the latest version of Spotify from the iOS App Store for this to work.

Here’s a link to an image, showing this at work.

If you use the Spotify app on iOS 13, give it a try.