I am trying to autofill column A from cell A2 until the last used cell in column B.

If Column B ends at row 12, fill from A2:A12; if it ends at row 96, fill from A2:A96...however, I cannot seem to figure out how to autofill such a range.

This is what I have so far:
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Dim filepath As Variant
Dim xltmp As Variant
Dim wb As Variant

'open the MS Excel worksheet
Set xltmp = New Excel.Application 'set MS Excel instance
filepath = txtFileName 'path of the file to be opened
xltmp.Visible = True 'open MS Excel
SetAttr filepath, vbNormal 'using this path
Set wb = xltmp.Workbooks.Open(filepath) 'open the workbook


'select column A, shift to right, insert column header, fill _
range with EmpID save workbook, close MS Excel.
xltmp.Columns("A:A").Select
xltmp.Selection.Insert Shift:=xlToRight
xltmp.Range("A1").Select
xltmp.ActiveCell.FormulaR1C1 = "EmpID"
xltmp.Range("A2").Select
xltmp.ActiveCell.FormulaR1C1 = txtEmpID.Value
xltmp.Selection.AutoFill Destination:=Range("A2:A3"), Type:=xlFillDefault
'xltmp.Range("A2:A3").Select
xltmp.ActiveWorkbook.Save
xltmp.ActiveWorkbook.Close
xltmp.Quit