要获取用户标识符，你可以使用Apple Music API中的用户授权流程。以下是一个使用OAuth 2.0进行用户授权并获取用户标识符的示例代码：

import UIKit
import AuthenticationServices

class ViewController: UIViewController, ASWebAuthenticationPresentationContextProviding {
    
    // Replace with your own client ID and redirect URI
    let clientID = "YOUR_CLIENT_ID"
    let redirectURI = "YOUR_REDIRECT_URI"
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        // Initiate the user authorization flow
        authorizeUser()
    }
    
    func authorizeUser() {
        // Create a URL for the authorization request
        guard let authURL = URL(string: "https://api.music.apple.com/v1/me") else {
            return
        }
        
        // Create a session configuration
        let configuration = ASWebAuthenticationSessionWebBrowserSessionConfiguration()
        configuration.presentationContextProvider = self
        
        // Create an authentication session
        let session = ASWebAuthenticationSession(url: authURL, callbackURLScheme: redirectURI) { (callbackURL, error) in
            // Handle the callback URL
            if let callbackURL = callbackURL {
                self.handleCallbackURL(callbackURL)
            }
        }
        
        // Start the authentication session
        session.start()
    }
    
    func handleCallbackURL(_ url: URL) {
        // Parse the callback URL to extract the authorization code
        let components = URLComponents(url: url, resolvingAgainstBaseURL: false)
        let queryItems = components?.queryItems
        let authorizationCode = queryItems?.first(where: { $0.name == "code" })?.value
        
        // Exchange the authorization code for a user token
        exchangeAuthorizationCode(authorizationCode)
    }
    
    func exchangeAuthorizationCode(_ authorizationCode: String?) {
        // Create a URL for the token exchange request
        guard let tokenURL = URL(string: "https://api.music.apple.com/v1/me/token") else {
            return
        }
        
        // Create the request body
        let requestBody = [
            "client_id": clientID,
            "code": authorizationCode,
            "grant_type": "authorization_code",
            "redirect_uri": redirectURI
        ]
        
        // Convert the request body to JSON data
        guard let requestData = try? JSONSerialization.data(withJSONObject: requestBody, options: []) else {
            return
        }
        
        // Create the token exchange request
        var request = URLRequest(url: tokenURL)
        request.httpMethod = "POST"
        request.setValue("application/json", forHTTPHeaderField: "Content-Type")
        request.httpBody = requestData
        
        // Send the token exchange request
        let task = URLSession.shared.dataTask(with: request) { (data, response, error) in
            // Handle the token exchange response
            if let data = data {
                self.handleTokenExchangeResponse(data)
            }
        }
        task.resume()
    }
    
    func handleTokenExchangeResponse(_ data: Data) {
        // Parse the token exchange response to extract the user token
        guard let json = try? JSONSerialization.jsonObject(with: data, options: []) as? [String: Any],
              let userToken = json["access_token"] as? String else {
            return
        }
        
        // Use the user token to make API requests on behalf of the user
        // Save the user token for future API calls
        
        print("User Token: \(userToken)")
    }
    
    func presentationAnchor(for session: ASWebAuthenticationSession) -> ASPresentationAnchor {
        return view.window ?? ASPresentationAnchor()
    }
}

注意：在上面的示例代码中，你需要将YOUR_CLIENT_ID替换为你自己的客户端ID，将YOUR_REDIRECT_URI替换为你自己的重定向URI。