Sub AddChkBox()
With ActiveDocument
  .FormFields.Add Range:=.Range.Characters.Last, Type:=wdFieldFormCheckBox
  .Characters.Last.Previous.Cut
  With .Content.Find
    .ClearFormatting
    .Text = "[_]{1,}"
    With .Replacement
      .ClearFormatting
      .Text = "^c"
    End With
    .Format = False
    .Forward = True
    .Wrap = wdFindContinue
    .MatchWildcards = True
    .Execute Replace:=wdReplaceAll
  End With
End With
End Sub