How to initialize Unmanaged CFString?

Question

Cocoa

Good afternoon.
I'm slowly learning Swift and got stuck on, it seems, little things, but I got stuck thoroughly.

Here is an example code:

import Foundation
import SystemConfiguration

var serviceID: Unmanaged<CFString> = .passRetained(NSString())
var userOptions: Unmanaged<CFDictionary> = .passRetained(NSDictionary())

let res = SCNetworkConnectionCopyUserPreferences(nil, &serviceID, &userOptions)
print("Res", res)
if res {
    print(serviceID.takeRetainedValue() as NSString)
    print(userOptions.takeRetainedValue() as NSDictionary)
}

No matter how hard I tried, res always comes with false. How does this bastard need to initialize the parameters in this case?