How do transfer mp3 files to iPhone that is configured with iCloud Music Library
I ran into this myself, came across your question while seeking the answer, but have since figured it out.

https://support.apple.com/en-us/HT204926

The key is to enable iCloud Music Library under Edit > Preferences > General in iTunes.  That then enables the "Add to iCloud Music Library" right click option over albums and individual files.

Items you "add to iCloud Music Library" will then automatically appear in the Music app on your iOS devices.