Xamarin.Forms - How to open sub menu after clicking on MenuItem in MasterDetailPage

Question

I am new to Xamarin.Forms. What I am trying to do here is to open pop-up with sub-items after clicking on the MenuItem.

I am using MasterDetailPage, and this is the code I have done so far. With this one, I have my sub-item menu always visible. Thank you.

            <StackLayout>
              <Label Text="Item1"/>

            <Label.Behaviors>

            </Label.Behaviors>
           </Label>
          <StackLayout Margin="0" Padding="20, 0, 0, 0">
            <Label Text="SubItem1" />
            <Label Text="SubItem2" />
          </StackLayout>

          <Label Text="Item2" >
        </StackLayout>

        </ContentPage>
      </MasterDetailPage.Master>

      <MasterDetailPage.Detail>
        <NavigationPage>
          <x:Arguments>
            <menuPages:MyPage />
          </x:Arguments>
        </NavigationPage>
      </MasterDetailPage.Detail>

    </MasterDetailPage>

MenuMaster.xaml.cs

namespace MenuMaster
{
    public partial class MainMenuMasterPage : MasterDetailPage
    {
        public MainMenuMasterPage()
        {
            InitializeComponent();

            //masterPage.ListView.ItemSelected += OnItemSelected;
        }
        void OnItemSelected(object sender, SelectedItemChangedEventArgs e) { }
    }
}