I think you should try something like this
<Window x:Class="WpfApplication1.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="MainWindow" Height="350" Width="525">
<Grid>
<Button HorizontalAlignment="Center"
VerticalAlignment="Center"
Height="20" Width="80"
ContextMenuService.Placement="Left"
ContextMenuService.HorizontalOffset="{Binding RelativeSource={RelativeSource Self}, Path=ActualWidth}"
ContextMenuService.VerticalOffset="{Binding RelativeSource={RelativeSource Self}, Path=ActualHeight}">
<Button.ContextMenu>
<ContextMenu>
<MenuItem Header="Menu item 01"/>
<MenuItem Header="Menu item 02"/>
</ContextMenu>
</Button.ContextMenu>
</Button>
</Grid>
</Window>