<Grid x:Name="PART_Grid">
    <Grid.ColumnDefinitions>
        <ColumnDefinition Width="*" />
        <ColumnDefinition Width="auto" />
    </Grid.ColumnDefinitions>

    <TextBox Grid.Column="0" />
    <Button Grid.Column="1" x:Name="PART_Button" Content="*" />
</Grid>

//Код по клику на кнопку; _grid = "PART_Grid"
var cMenu = new ContextMenu();
cMenu.Items.Add(new MenuItem { Header = "123", Items = { new MenuItem { Header = "hello" } } });
cMenu.Items.Add(new MenuItem { Header = "123123" });
cMenu.Items.Add(new MenuItem { Header = "123123123" });
Point pt = _grid.TransformToVisual(Application.Current.MainWindow).Transform(new Point(0, 0));
Canvas.SetLeft(cMenu, pt.X);
Canvas.SetTop(cMenu, pt.Y);
//cMenu.Width = _grid.ActualWidth;
//cMenu.HorizontalOffset = pt.X;
//cMenu.VerticalOffset = pt.Y + _grid.ActualHeight;
cMenu.IsOpen = true;

var cMenu = new ContextMenu();
cMenu.Items.Add(new MenuItem { Header = "123", Items = { new MenuItem { Header = "hello" } } });
cMenu.Items.Add(new MenuItem { Header = "123123" });
cMenu.Items.Add(new MenuItem { Header = "123123123" });

cMenu.Placement = System.Windows.Controls.Primitives.PlacementMode.Relative;
cMenu.PlacementTarget = this;
cMenu.PlacementRectangle = new Rect(0, this.ActualHeight, 30, 30);
cMenu.Width = this.ActualWidth;
cMenu.IsOpen = true;