When browsing your library, you’ll eventually see a list of tracks. For instance, if you open a playlist or you enter a particular album.
You can then interact with an individual track to see available options. The method can depend on the platform you’re using to control your Sonos (e.g. mobile app vs a desktop app), but it should be familiar to you from other Sonos services. For instance, in a mobile app, you can long-press on an item or click the “triple-dot” menu item for it.
That will bring up the initial menu. From there, choose the More… or Info & Options type item to access additional actions. Here, you can Start Plex Mix.
Choosing that will immediately start a Plex Mix using the normal “radio” behavior from Sonos.
Related Page: Playback