Button b = new Button()
b.HorizontalAlignment = HorizontalAlignment.Left;
b.VerticalAlignment = VerticalAlignment.Top;
b.Margin.Top = [твоя позиция по y];
b.Margin.Left = [твоя позиция по x];
b.Width = [ширина];
b.Height = [высота];
grd.Children.Add(b)