AppClass.cs

public class AppClass {
  private User _mainUser;
  
  public bool Login() {
    Console.Clear();
    return User.Login(_mainUser);
  }
}

user.cs

public class User {
  public int Id { get; set; }
  public string Username {get;set;}
  public string Password {get;set;}
  
  public static Login(User us) {
    //ask user for Id
    //ask user for pw
    int userId = 0;
    Int32.TryParse(Console.ReadLine(), out userId);
    
    Console.WriteLine("Enter your pw: ");
    var pw = Console.ReadLine();
    
    return us.Id = userId && us.Password = pw;
  }
}