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 that item 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.
That will bring up the initial menu. From there, choose the More... or Info & Options type item to access additional options. Here, you can Start Plex Mix Radio.
Choosing that will immediately start a Plex Mix using the normal "radio" behavior from Sonos.
Related Page: Sonos > Playback