Ios swift – Request authorization called by POST method to Last.fm API using swift

I’m trying to follow Last.fm’s tutorial to consume their API in an iOS app (using Swift), but I don’t know what’s wrong with my code. They ask to do something called Last.fm method signature which I’m not understanding how to insert into my program. Here is the link to the tutorial I try to follow: https://www.last.fm/api/mobileauth

Here is my current code:

import UIKit

struct LoginRequestBody: Codable { 
    var username: String 
    var password: String 
    var api_key: String 
    var api_sig: String 
}

enum AuthenticationError: Error { 
    case invalidCredentials 
    case custom(errorMessage: String) 
}

class APIService {
    func requestAPI(username: String, password: String, api_key: String, api_sig: String) {
        guard let url = URL(string: "http://www.last.fm/api/auth/?api_key=xxx") else {
            return
        }
        
        var urlRequest = URLRequest(url: url)
        urlRequest.httpMethod = "POST"
        
        let body = LoginRequestBody(username: username, password: password, api_key: api_key, api_sig: api_sig)
        let bodyStr = [
            "username": "(body.username)",
            "password": "(body.password)",
            "api_key": "(body.api_key)",
            "api_sig": "(body.api_sig)"]
        
        urlRequest.httpBody = try? JSONSerialization.data(withJSONObject: bodyStr, options: .fragmentsAllowed)
        
        let task = URLSession.shared.dataTask(with: urlRequest) { data, _, error in
            guard let data = data, error == nil else {
                return
            }
            
            do {
                let response = try JSONSerialization.jsonObject(with: data, options: .fragmentsAllowed)
                print(response)
            } catch {
                print(error)
            }
        }
        task.resume()
    }
}