Вы можете использовать Path
:
<ToggleButton x:Name="toggleButton" Content="Button" Margin="100" />
<Popup IsOpen="{Binding IsChecked, ElementName=toggleButton}"
PlacementTarget="{Binding ElementName=toggleButton}"
Placement="Right"
StaysOpen="False"
AllowsTransparency="True"
VerticalOffset="-90">
<Grid>
<Border Background="Blue" BorderThickness="1" BorderBrush="Black"
Width="200" Height="200" Margin="24,0,0,0">
<TextBlock HorizontalAlignment="Center" VerticalAlignment="Center" Foreground="White">...</TextBlock>
</Border>
<Path StrokeThickness="1" Stroke="Black" Fill="Blue" VerticalAlignment="Center">
<Path.Data>
<PathGeometry>
<PathFigure StartPoint="25,0">
<PolyLineSegment Points="0,25 25,50"/>
<LineSegment Point="25,0" IsStroked="False"/>
</PathFigure>
</PathGeometry>
</Path.Data>
</Path>
</Grid>
</Popup>