How to sync iTunes to an already set up iPhone

Hi,

I bought a new iPhone 8 a week ago, and I've set it up as a new phone. I didn't use iTunes to do this. Now, I want to get all of my music on my phone. When I connect my iPhone to my macbook, iTunes pops up with a question whether I want to set my iPhone up as a new phone or restore from a backup (the only choice is the backup from my iPod). If I understand this correctly, I will lose all of my files, settings, contacts, etc. if I choose one of these.

How to I get my music on my iPhone?

Thanks.