When you open up the Pandora app in iMessage, you'll see whatever song you're currently playing as well as a a few different lists of songs, including top trending tunes and a couple categories ("I like you" and "rage" were two of the more diametrically opposed options). There's also a search bar so you can find a specific tune to send to your friend. Naturally, because this is an iMessage app, Pandora also included stickers. There are a few goofy emoji faces, some Pandora logos and some thumbs-up and down options.
Music sharing was demoed by Apple back when iMessage apps launched, using Apple Music of course. But Apple Music lets you hear entire songs (provided the recipient is a subscriber), while Pandora is stuck with only samples for now. That said, Pandora appears to be the first major music service beyond Apple Music to embrace iMessage, so that's better than nothing we suppose. And Pandora will be set to go when its long-awaited on-demand streaming service is ready to go.