How can I retrieve the configuration values in a MainViewModel system

In WPF I have created a form "Configuration.xaml" to enter the configuration values of my program. It contains a DevExpress "Hamburguermenu" control so that, when clicking on each menu option, different subforms appear, such as "Configuration_General.xaml", with the corresponding TextBox and ComboBox values.

The DevExpress "Hamburguermenu" control introduces the different menu options using the "MainViewModel.cs" system.

"Configuration.xaml" contains a "Save" button that starts the subform "configuracion_save.xaml" which is where there is a process that must collect the TextBox and ComboBox values from each subform in order to store them in a file.

The problem is, I don't know how to retrieve the TextBox and ComboBox values from each subform in order to save them.

configuracion.xaml
---------------------
            xmlns:dx="http://schemas.devexpress.com/winfx/2008/xaml/core"
            xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
            xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
            xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
            xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
            xmlns:dxwui="http://schemas.devexpress.com/winfx/2008/xaml/windowsui"
            xmlns:dxwuin="http://schemas.devexpress.com/winfx/2008/xaml/windowsui/navigation"
            xmlns:dxmvvm="http://schemas.devexpress.com/winfx/2008/xaml/mvvm"
            xmlns:local="clr-namespace:Name_MainWindow"
            mc:Ignorable="d"
            WindowStartupLocation="CenterScreen"
            Loaded="Fcn_OnWindowLoaded" Closing="Fcn_WindowClosing" Closed="Window_Closed"
            Title="Configuración" dx:ThemeManager.ThemeName="Office2016ColorfulSE" Width="850" Height="500">
       
       
           
       
       
       
           
               
                   
                   
                   
               
           
       
   
                       ItemsSource          = "{Binding HamburgerMenuItems}"
               BottomBarItemsSource = "{Binding HamburgerMenuBottomBarItems}"
               IsMenuVisible        = "{Binding IsMenuVisible}"
               SelectedItem         = "{Binding SelectedItem, Mode=TwoWay}"
               NavigationTarget     = "{Binding ElementName=service}">
           
               
                   
                       
                   
               
           
       
   

MainViewModel.cs
---------------------
namespace Name_MainWindow
{  public class MainViewModel : DevExpress.Mvvm.NavigationViewModelBase
    {   public System.Collections.ObjectModel.ReadOnlyCollection HamburgerMenuItems { get; }
        public System.Collections.ObjectModel.ReadOnlyCollection HamburgerMenuBottomBarItems { get; }
        public bool IsMenuVisible
        {   get{ return GetProperty(() => IsMenuVisible); }
            set{ SetProperty(() => IsMenuVisible, value); }
        }
        public IHamburgerMenuItemViewModel SelectedItem
        {   get{ return GetProperty(() => SelectedItem); }
            set{ SetProperty(() => SelectedItem, value, OnSelectedItemChanged); }
        }

        public MainViewModel()
        {  
            HamburgerMenuItems          = new System.Collections.ObjectModel.ReadOnlyCollection(InitializeMenuItems());
           
            HamburgerMenuBottomBarItems = new System.Collections.ObjectModel.ReadOnlyCollection(InitializeBottomBarItems());

            SelectedItem  = HamburgerMenuItems[0];
            IsMenuVisible = true;
        }

        protected virtual System.Collections.Generic.IList InitializeMenuItems()
        {   string  Mensaje                 = null;
            var     IHamburgerMenuItem_List = new System.Collections.Generic.List();

            // --------------------
            // HOME.
            // --------------------
            Mensaje = "Home";
            IHamburgerMenuItem_List.Add(new NavigationItemModel(Mensaje){ NavigationTarget = typeof(Name_MainWindow_Configuracion.MainWindow_Configuracion_Inicio), Glyph = "../images/configuracion/configuracion_home.png" });
            IHamburgerMenuItem_List.Add(new SeparatorItem());

            // --------------------
            // GENERAL.
            // --------------------
            Mensaje = "General";
            IHamburgerMenuItem_List.Add(new NavigationItemModel(Mensaje){ NavigationTarget = typeof(Name_MainWindow_Configuracion.MainWindow_Configuracion_General), Glyph = "../images/configuracion/configuracion_general.png" });
            IHamburgerMenuItem_List.Add(new SeparatorItem());

            // --------------------
            // SAVE.
            // --------------------
            Mensaje = "Save";
            IHamburgerMenuItem_List.Add(new NavigationItemModel(Mensaje){ NavigationTarget = typeof(Name_MainWindow_Configuracion.MainWindow_Configuracion_Save), IsAlternatePlacementItem = true, Glyph = "../images/configuracion/configuracion_save.png" });

            return IHamburgerMenuItem_List;
        }

        protected virtual System.Collections.Generic.IList InitializeBottomBarItems()
        {
            return new System.Collections.Generic.List()
            {   new BottomBarNavigationItemModel()       { NavigationTarget = typeof(Name_MainWindow_Configuracion.MainWindow_Configuracion_Guardar),
                                                           Glyph = "../images/configuracion/configuracion_guardar.png", IsAlternatePlacementItem = true },
                new BottomBarCheckableItemModel()        { Glyph = "check.png" },
                new BottomBarRadioItemModel("RadioGroup"){ Glyph = "Icons/Radio1.png" },
                new BottomBarRadioItemModel("RadioGroup"){ Glyph = "Icons/Radio2.png" }
            };
        }
        void OnSelectedItemChanged()
        {   if (SelectedItem != null)
            {   // IsMenuVisible = HamburgerMenuSelectedItem.Caption != "Simple Page";
            }
        }
    }
}

Configuracion_General.xaml
-------------------------------
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
             Background="White"
             mc:Ignorable="d"
             d:DesignWidth="650" d:DesignHeight="450">
   
       
           
               
               
               
           
       
   

   
       
           
               
               
                   
                   

                   
                                                     TextWrapping="Wrap" AcceptsReturn="False" HorizontalAlignment="Left" VerticalAlignment="Top" VerticalContentAlignment="Top"/>
               
           

       
   


Configuracion_save.cs
------------------------
namespace Name_MainWindow_Configuracion
{
    public partial class MainWindow_Configuracion_Guardar : System.Windows.Controls.UserControl
    {  
        public MainWindow_Configuracion_Guardar()
        {    InitializeComponent();
             this.Page_Load();
        }

        protected void Page_Load()
        {    bool Fcn_Correcto = Fcn_AppDatos();
        }

        private bool Fcn_AppDatos()
        {   bool Fcn_Correcto = false;

            // AT THIS POINT I have to retrieve the values from each subform so that I can save them to a file.

            // For example, in the form "general_configuration.cs" I have to retrieve the TextBox from the "User_TextBox" field.

           
            return Fcn_Correcto;
        }
    }
}