You should make more one window with button, like this in XAML:
<Window AllowTransparency="True" ResizeMode="NoResize" WindowStyle="None"
Topmost="True" ShowInTaskbar="False" SizeToContent="WidthAndHeight"
Background="{x:Null}">
<Button x:Name="HideShowButton" Content="Restore" />
</Window>
Button cannot exist without window, but with these properies window will be invisible.