Copy Formula Down Based On Last Cell In Another Column

i want to insert value from the name of file from this formula "=MID(CELL(""filename""), FIND(""["",CELL(""filename""))+1,10)" to range b2 until last empty range that i found form this formula =ROW(OFFSET(A1, COUNTA(A:A)-1,0))

Sub lumu()
Dim x As Variant
x = "=ROW(OFFSET(A1,COUNTA(A:A)-1,0))"
Columns("B:B").Select
Selection.Insert Shift:=xlToRight, CopyOrigin:=xlFormatFromLeftOrAbove
Range("B1").Select
ActiveCell.FormulaR1C1 = "kode_wilayah"
Range("B2").Select
ActiveCell.FormulaR1C1 = _
"=MID(CELL(""filename""),FIND(""["",CELL(""filename""))+1,10)"
Range("B2").Select
Selection.AutoFill Destination:=Range("B2", Range("B" & x)), Type:=xlFillDefault 'this an error
Range("B2:B123").Select
On Error Resume Next
ActiveSheet.Name = "t"
End Sub