I write it
Grid grid = new Grid();
grid = App.Current.Windows[1].FindName("grid1") as Grid;
if (grid != null)
{
RowDefinition rd = new RowDefinition();
rd.Height = GridLength.Auto;
grid.RowDefinitions.Add(rd);
TextBox txt = new TextBox();
txt.Name = "txtNewAdd";
txt.Height = 23;
txt.VerticalAlignment = VerticalAlignment.Top;
TextBox txtName = new TextBox();
txtName= grid.FindName("txtFname") as TextBox;
if (txtName!=null)
{
txt.Background = txtName.Background ;
}
Grid.SetColumn(txt, 1);
Grid.SetRow(txt,(grid.RowDefinitions.Count-1));
grid.Children.Add(txt);
}