<ListBox x:Name="listboxFolder" BorderThickness="0" ItemsSource="{Binding Path=ClientsOnlineList, UpdateSourceTrigger=PropertyChanged}" Margin="0,50,0,10" HorizontalAlignment="Left" Width="301" Background="#FFD0E6C5" >
            <ListBox.ItemContainerStyle>
                <Style TargetType="{x:Type ListBoxItem}">
                    <EventSetter Event="PreviewMouseLeftButtonDown" Handler="SelectClient"></EventSetter>
                    <Setter Property="Margin" Value="5"/>
                    <Setter Property="Padding" Value="5"/>
                    <Setter Property="Cursor" Value="Hand"/>
                    <Style.Triggers>
                        <Trigger Property="IsSelected" Value="True">
                            <Setter Property="Foreground" Value="Green"/>Bold"/>
                            <Setter Property="FontSize" Value="23"/>
                        </Trigger>
                    </Style.Triggers>
                </Style>
            </ListBox.ItemContainerStyle>
            <ListBox.ItemTemplate >
                <DataTemplate >
                    <TextBlock Text="{Binding UserName}"  />
                </DataTemplate>
            </ListBox.ItemTemplate>
        </ListBox>

/// <summary>
    /// Это коллекция для заполнения ListBox
    /// </summary>
    private ObservableCollection<Animal> _Animals;
    public ObservableCollection<Animal> Animals
    {
        get { return _Animals; }
        set
        {
            if (_Animals == value) return;
            _Animals = value;
            PropertyChanged(this, new PropertyChangedEventArgs(nameof(Animals)));
        }
    }

    /// <summary>
    /// Текущее выбранное животное в ListBox
    /// </summary>
    private Animal _CurrentAnimal;
    public Animal CurrentAnimal
    {
        get { return _CurrentAnimal; }
        set
        {
            if (_CurrentAnimal == value) return;
            _CurrentAnimal = value;
            PropertyChanged(this, new PropertyChangedEventArgs(nameof(CurrentAnimal)));
         }
    }

<ListBox ItemsSource="{Binding Animals, Mode=TwoWay}"
           SelectedItem="{Binding CurrentAnimal}"/>

private void SelectClient(object sender, MouseEventArgs e)
    {
        var index = listboxFolder.Items.IndexOf(((ListBoxItem)sender).DataContext);
        MessageBox.Show(index.ToString());
    }

listboxFolder.SelectionChanged += (sender, e) => {
    var id = (listboxFolder.SelectedItem as [Ваш класс клиента]).id;
    MessageBox.Show(id.ToString());
};