Record yourself using the View ► Freeze Panes ► Freeze Top Row command and this is what you get for .FreezePanes.
With ActiveWindow
If .FreezePanes Then .FreezePanes = False
.SplitColumn = 0
.SplitRow = 1
.FreezePanes = True
End With
So modifying the .SplitColumn and/or .SplitRow properties should do it for you regardless on what the ActiveCell property is.