you can put the window buttons directly in your main window like this
<Grid>
<!-- the window button commands -->
<Controls:WindowButtonCommands Panel.ZIndex="1"
HorizontalAlignment="Right"
VerticalAlignment="Top"
Height="{Binding TitlebarHeight, Mode=OneWay, RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type Controls:MetroWindow}}}" />
...
</Grid>
hope that helps