Auto-Number for PO spreadsheet

Try this:-
Right click sheet tab, Select "View Code", Vbwindow appears, Past code in window, Close Vb window
Code runs when you click "A4"

Private Sub Worksheet_SelectionChange(ByVal Target As Range)
If Target.Address(0, 0) = "A4" Then
    Target.EntireRow.Insert
    Target.Offset(-1) = IIf(Target = "", 2016000, Target + 1)
End If
End Sub