<Button
            Clicked="ShowPopup"
            HorizontalOptions="Center"
            Text="Show RadPopup"
            VerticalOptions="Start"
            x:Name="button">
            <telerik:RadPopup.Popup>
                <telerik:RadPopup
                    IsModal="True"
                    OutsideBackgroundColor="#6F000000"
                    x:Name="popup">
                    <telerik:RadBorder BackgroundColor="Wheat" CornerRadius="8">
                        <Grid
                            HeightRequest="200"
                            Padding="10"
                            WidthRequest="200">
                            <Grid.RowDefinitions>
                                <RowDefinition Height="30" />
                                <RowDefinition Height="auto" />
                            </Grid.RowDefinitions>
                            <telerik:RadButton
                                AutomationId="CloseButton"
                                Clicked="ClosePopup"
                                Grid.Row="0"
                                HorizontalOptions="End"
                                Padding="2"
                                Text="Close" />
                            <telerik:RadComboBox AutomationId="staticItemsComboBox" Grid.Row="1">
                                <telerik:RadComboBox.ItemsSource>
                                    <x:Array Type="{x:Type x:String}">
                                        <x:String>USA</x:String>
                                        <x:String>Uganda</x:String>
                                        <x:String>Ukraine</x:String>
                                        <x:String>Canada</x:String>
                                        <x:String>France</x:String>
                                        <x:String>Italy</x:String>
                                        <x:String>United Kingdom</x:String>
                                        <x:String>China</x:String>
                                        <x:String>Japan</x:String>
                                    </x:Array>
                                </telerik:RadComboBox.ItemsSource>
                            </telerik:RadComboBox>
                        </Grid>
                    </telerik:RadBorder>
                </telerik:RadPopup>
            </telerik:RadPopup.Popup>
        </Button>

        private void ClosePopup(object sender, EventArgs e)
        {
            popup.IsOpen = false;
        }
        private void ShowPopup(object sender, EventArgs e)
        {
            popup.IsOpen = true;
        }

 <local:ReminderTemplateSelector.TaskTemplate>
     <DataTemplate>
         <telerik:CalendarBorderLabel Text="{Binding Text}"
                                      TextColor="{StaticResource AccentColor4}"
                                      HorizontalTextAlignment="Center"
                                      VerticalTextAlignment="Center"
                                      FontAttributes="Bold">
             <Label FontFamily="TelerikFontExamples"
                    Text="&#xe80b;"
                    TextColor="{StaticResource AccentColor4}"
                    HorizontalTextAlignment="Center"
                    VerticalTextAlignment="End"
                    Margin="{OnPlatform Default=0, WinUI='0, 0, 0, 4'}" />
         </telerik:CalendarBorderLabel>
     </DataTemplate>
 </local:ReminderTemplateSelector.TaskTemplate>

public class ReminderTemplateSelector : DataTemplateSelector
{

    public static readonly BindableProperty TasksProperty = BindableProperty.Create(nameof(Tasks), typeof(ReminderTemplateSelector), typeof(ReminderTemplateSelector), null);

    private List<DateTime> tasks;
    public ReminderTemplateSelector()
    {
      
    }

    public DataTemplate TaskTemplate { get; set; }

    public List<DateTime> Tasks
    {
        get => this.tasks;
        set
        {
            this.tasks = value;
        }
    }

    protected override DataTemplate OnSelectTemplate(object item, BindableObject container)
    {
        var node = item as CalendarNode;
        var date = node.Date.Value;
          
        bool hasTaskReminder = false;
        if (this.tasks != null)
        {
            hasTaskReminder = this.tasks.Contains(date);
        }
         

        if (hasTaskReminder)
        {
            return this.TaskTemplate;
        }
         
        return null;
    }
}

        private async void OnDisplayDateChanged(object sender, ValueChangedEventArgs<System.DateTime> e)
        {

            this.ReminderTemplateSelector.Tasks = LoadTask(e.NewValue);
        }