One way to access the shared files folder that is used in some projects is to let the IIS application pool use the identity of the user that has access to that shared folder. That won’t work now when we are using the Kestrel web server.
Instead you can map that shared folder as a Network drive and reference that in appsettings.json. In Windows you right click in the explorer and select Add network location, on the mac you open Finder, select Go in the menu and click on Connect to server.
"Folder": {
"Local": "..\\Files",
"Shared": "\\\\network.path\\sharedfiles\\customer\\Files"
},