Switch from Cells to Range - excel

Question

I see Range.AutoFill method (Excel) example:
Set sourceRange = Worksheets("Sheet1").Range("A1:A2")
Set fillRange = Worksheets("Sheet1").Range("A1:A20")
sourceRange.AutoFill Destination:=fillRange
I want to input a formula into the first empty column (always row 2) and then copy that down for all rows in the register (it's data copied from an outside source).
Sub SetNextEmptyFormula(strFormula As Variant)
Dim ws As Worksheet
Dim lCol As Long, lRow As Long
Set ws = Workbooks("myworkbook.xlsm").Worksheets("register")
' Always row 2 -> lRow, always in lCol
lCol = ws.Range("A1").End(xlToRight).Column
lRow = ws.Range("A1").End(xlDown).Row
ws.Cells(2, lCol + i).Value = CStr(strFormula)
Set sourceRange = ws.Range(???)
Set fillRange = ws.Range(???)
sourceRange.AutoFill Destination:=fillRange
End Sub

Answer 1

I'm not sure how you're going to build your formula, but if you're just looking for help referencing these cells, this will do:
Option Explicit
Sub UseSub()
Dim myFormula
myFormula = "=ROW()"
Call SetNextEmptyFormula(myFormula)
End Sub
Sub SetNextEmptyFormula(strFormula As Variant)
Dim WS As Worksheet
Dim lCol As Long, lRow As Long
Dim fillRange As Range
Set WS = Workbooks("myworkbook.xlsm").Worksheets("register")
'Set WS = Workbooks("Book1.xlsm").Worksheets("register")
With WS
' Always row 2 -> lRow, always in lCol
lCol = .Range("A1").End(xlToRight).Column
lRow = .Range("A1").End(xlDown).Row
.Cells(2, lCol + 1).Formula = CStr(strFormula)
'FillDown Only requires you reference the whole range Once, You only need "FillRange"
'Set sourceRange = ws.Range(???)
Set fillRange = .Range(.Cells(2, lCol + 1), .Cells(lRow, lCol + 1))
fillRange.FillDown
End With
End Sub