Mac keeps wanting to setup iphone as new iphone

My wife's iphone (12 pro) has been setup without a computer and is associated with her appleID. We have an iMac that is associated with my appleID. I am trying to add music to her phone that is on the iMac's Music Library. When I connect my wife's iPhone to the iMac and navigate to it in Finder, I am asked to either setup the phone as a new iPhone or restore from a backup. I want to do neither and just want to put music on my phone... Don't know what to do. Really don't want to wipe the phone just to get 12 songs on it.