how do I send a song from my music on iPhone iOS 10.3.2 to messenger

If you're on an iOS device, when song is playing on the Music app > click on 3 small dots (bottom right) > you'll see a menu brought up with an option to Share Song > next menu, choose Messages.

How to send a song to someone via messenger or text from my music play list on iOS 10.3.2

To be clear... you can't actually share the song file itself. Only a link to where it can be purchased.

iTunes purchases belong to only the purchaser.