VBA calculate checksum xor of string

Maybe :

Sub Test()
  Dim cell As Range, chk As Long, i As Long, str1 As String
  For Each cell In Range("A2:A4")
      str1 = cell.Value
      chk = Asc(Left$(str1, 1))
      For i = 2 To Len(str1)
          chk = chk Xor Asc(Mid$(str1, i, 1))
      Next i
      cell.Offset(, 3).Value = cell.Value & Right$("00" & Hex(chk), 2)
  Next cell
End Sub