you may have a stack panel or grid as a child control and place more contents on the panel or grid controls. The following code snippet creates a stack panel and places some contents on it.
<Popup Margin="10,10,0,13" Name="Popup1" HorizontalAlignment="Left"
VerticalAlignment="Top" Width="194" Height="200" IsOpen="True">
<StackPanel>
<TextBlock Name="McTextBlock"
Background="LightBlue" >
This is popup text
</TextBlock>
<Button Content="This is button on a Pupup" />
</StackPanel>
</Popup>