Custom design of a popup in wpf using c# (telerik )

Question

I am showing an pop up in a button click event> I have kept a custom user control inside that popup and so whenever the pop is being shown taht custom usercontrol will be displayed.But now i need to modify the custom control . Current custom control inside popup looks like below

but i need the corner to be look like the below image.. How to do it.? I need that sharp corner in my control and i need that sharp corner point whenever the popup been shown

bolow the code of the button and its popup

 <telerik:RadButton Name="btnH" Grid.Column="1" HorizontalAlignment="Left" Margin="444,56,0,0" Grid.Row="2" VerticalAlignment="Top" 
                 Width="23" Height="23" BorderThickness="6" BorderBrush="#4E4E4E" Click="btnH_Click" >
            <Image Source="Images/help.png" />
        </telerik:RadButton>
        <Popup PopupAnimation="Fade" Placement="Mouse" AllowsTransparency="True" StaysOpen="False" x:Name="TooltipPopup" >
            <Border Background="AntiqueWhite" CornerRadius="0" BorderThickness="1">
                <StackPanel Margin="1" Orientation="Horizontal" >
                    <local:UCToolTip></local:UCToolTip>
                </StackPanel>
            </Border>
        </Popup>

Answer 1

Here is an example in XAML:

<Grid Width="300" Height="200">
    <Grid.RowDefinitions>
        <RowDefinition/>
        <RowDefinition Height="Auto"/>
    </Grid.RowDefinitions>
    <Grid Background="Orange">
        <!--Your content here-->
    </Grid>
    <Polygon Grid.Row="1"
        Points="0,0 15,0, 0,30" Stroke="Orange" Fill="Orange" />
</Grid>

Answer 2

In your control for the tooltip, you can use a Grid to draw two things on top of each other.

The first, a Canvas with a custom PathGeometry of the shape you want, this is a good reasource:

https://msdn.microsoft.com/en-us/library/ms747393(v=vs.100).aspx

Then, make the background transparent with what you currently have and have it occupy the same cell in the Grid, so it appears on top of the shape.