If you have an Apple Music account and have sync'd your Macbook library to the cloud, then dragging a song into the Music app should automatically add it to your phone as well. If not, go to Music on your Macbook > File > Library > Update Cloud Library. That should sync the two libraries together. i.e. music added via your phone will download to your Macbook, and anything on the Macbook not on your phone will upload to it.
That's pretty much it in a nutshell. It kind of works OK, but if you have a big local library, syncing that to the cloud is buggy as ****, so back up your library before going anywhere near the sync options!