Break down how this is achieved. You using a 6S+? Cos for me, once Apple Music is enabled, I cannot add MP3s onto my phone from my Macbook nor my HDD.
I got a regular 6S, doesn't matter the model of iPhone tho.
In iTunes on your Mac go to Settings & check off iCloud Music Library under General.
Then on your iPhone go to settings>>Music and under Library enable iCloud Music Library.
It'll upload all of your music to the cloud. I'ma warn you tho, this shyt still kinda in beta, so when I did it it kinda fukked up some of my albums and added clean versions of some albums/song. (Google Play Music did the same shyt) It was worth it for me tho.