How can i display the alert on Master detail page when i navigate to Master detail page

I'm using a master-detail page and tabbed page on one screen. I need to display an alert when I navigate to the master-detail page. That an alert should display the complete page of the master-detail and tabbed page. I don't wanna operate any function until I click on the OK button of an alert.

**Master-Detail Page CS Code: **

public partial class MainMasterDetailPage : MasterDetailPage
    {

        public MainMasterDetailPage()
        {
            InitializeComponent();

            NavigationPage.SetHasNavigationBar(this, false);
            navigationDrawerList.ItemsSource = GetMasterPageLists();
            this.IsPresented = false;
            Detail = new NavigationPage(new DashboardTabbedPage());
        }

        private void OnMenuSelected(object sender, SelectedItemChangedEventArgs e)
        {
            var item = (MasterPageList)e.SelectedItem;

            if (item.Title == "Settings")
            {
                // Detail.Navigation.PushAsync(new SettingPage());
               // IsPresented = false;
            }
            else
            {
                string dbPath = Path.Combine(Environment.GetFolderPath(Environment.SpecialFolder.Personal), "vssSQLite.db3");
                SQLiteConnection db = new SQLiteConnection(dbPath);
                db.DropTable<LoginSqlLiteM>();

                Application.Current.MainPage.Navigation.PushAsync(new MainPage());
            }

        }
        List<MasterPageList> GetMasterPageLists()
        {
            return new List<MasterPageList>
            {
                new MasterPageList() { Title = "Settings", Icon = "settings.png" },
                new MasterPageList() { Title = "Logout", Icon = "logout.png" },

            };
        }

    }
}

Master-Detail page XAML code:

<?xml version="1.0" encoding="utf-8" ?>
<MasterDetailPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
                  xmlns:local="clr-namespace:VSS.MasterPages"
                  xmlns:local1="clr-namespace:VSS.Pages"
                  IsPresented="False"
             x:Class="VSS.MasterPages.MainMasterDetailPage">

    <MasterDetailPage.Master>

        <ContentPage Title="☰">
            <StackLayout BackgroundColor="#ffffff">
                <StackLayout Margin="25,10,25,0" Padding="0,15,0,15">
                    <Image Source="VSSIcon.png" HorizontalOptions="Center" HeightRequest="80" WidthRequest="80"></Image>
                    <Label Text="Sched.ly" TextColor="#1CA9FF" FontSize="Large"  HorizontalOptions="CenterAndExpand"></Label>
                    <Label Text="Always on Time " TextColor="#1CA9FF" FontSize="Large" FontAttributes="Italic"  HorizontalOptions="CenterAndExpand"></Label>
                </StackLayout>
                <ListView x:Name="navigationDrawerList"  
                  RowHeight="60"  
                  SeparatorVisibility="None"  
                  ItemSelected="OnMenuSelected"  
                  BackgroundColor="#2196F3">
                    <ListView.ItemTemplate>
                        <DataTemplate>
                            <ViewCell>
                                <StackLayout VerticalOptions="FillAndExpand"  
                                 Orientation="Horizontal"  
                                 Padding="20,10,0,10"  
                                 Spacing="20">

                                    <Image Source="{Binding Icon}"  
                                     WidthRequest="30"  
                                     HeightRequest="30"  
                                     VerticalOptions="Center" />
                                    <Label Text="{Binding Title}"  
                                     FontSize="Medium"  
                                     VerticalOptions="Center"  
                                     TextColor="White"/>
                                </StackLayout>
                            </ViewCell>
                        </DataTemplate>
                    </ListView.ItemTemplate>
                </ListView>
            </StackLayout>
        </ContentPage>

    </MasterDetailPage.Master>

    <MasterDetailPage.Detail>
        <NavigationPage>
            <x:Arguments>
                <local:DashboardTabbedPage/>
            </x:Arguments>
        </NavigationPage>
    </MasterDetailPage.Detail>
</MasterDetailPage>

Please find the master-detail and Tabbed page image:

Currently, I'm displaying the alert only inside of the tabbed page...but I want to display the alert for total master-detail page means doesn't wanna allow any action before click on the OK button of alert.
For this, I want to use Absolute layout and display the alert for overall Absolute layout. How can I use this code in the master-detail page?

Alert code for Absolute Layout:

<AbsoluteLayout x:Name="vendorDashboardLayout" HorizontalOptions="CenterAndExpand">
    <ContentView x:Name="overlay"
                         AbsoluteLayout.LayoutBounds="0, 0, 1, 1"
                         AbsoluteLayout.LayoutFlags="All"
                         BackgroundColor="#C0808080">
        <StackLayout HorizontalOptions="Center" VerticalOptions="Center">

            <StackLayout x:Name="Alert"   BackgroundColor="#D2D7DD"  HorizontalOptions="StartAndExpand" VerticalOptions="StartAndExpand"  HeightRequest="220"  WidthRequest="350" >
                <StackLayout  HorizontalOptions="Fill" >
                    <Frame  BackgroundColor="#375587" HeightRequest="20" BorderColor="#015198">
                        <Label  Text="Alert!" HorizontalTextAlignment="Start" VerticalTextAlignment="Center"
                                    FontSize="14.5" TextColor="White">
                        </Label>
                    </Frame>
                </StackLayout>
                <StackLayout Margin="10,30,10,20">
                    <Label x:Name="alertMessage" WidthRequest="330"  HeightRequest="100" TextColor="Black"  />
                </StackLayout>

                <StackLayout HorizontalOptions="EndAndExpand"  Orientation="Horizontal" Margin="10,20,20,20" >
                    <Button Text="OK" BorderRadius="6" BackgroundColor="#375587" TextColor="White" HorizontalOptions="EndAndExpand" Clicked="confirm_Btn" >
                        <Button.WidthRequest>
                            <OnPlatform x:TypeArguments="x:Double">
                                <On Platform="Android" Value="80" />
                                <!--<On Platform="iOS" Value="60" />-->
                            </OnPlatform>
                        </Button.WidthRequest>
                        <Button.FontSize>
                            <OnPlatform x:TypeArguments="x:Double">
                                <On Platform="Android" Value="13" />
                                <!--<On Platform="iOS" Value="13" />-->
                            </OnPlatform>
                        </Button.FontSize>
                        <Button.HeightRequest>
                            <OnPlatform x:TypeArguments="x:Double">
                                <On Platform="Android" Value="40" />
                                <!--<On Platform="iOS" Value="30" />-->
                            </OnPlatform>
                        </Button.HeightRequest>
                    </Button>
                </StackLayout>
            </StackLayout>
        </StackLayout>
    </ContentView>

</AbsoluteLayout>