无法在WPF DataGrid中选择第一行

I have a WPF Datagrid with 3 columns as part of a window. The ItemsSource is set from an array of payment objects (grdPayments.ItemsSource = payments). When the focus is set to the grid in code behind I want the third cell in the first row to get focus.I have a WPF Datagrid with 3 columns as part of