Sub OptTxtFmFld()
Dim Prot As Variant, BmkRng As Range
Const Pwd As String = "" 'Insert password here
Const BmkNm As String = "BkMk" ' The bookmark name
With ActiveDocument
  Prot = .ProtectionType
  If .ProtectionType <> wdNoProtection Then
    Prot = .ProtectionType
    .Unprotect Password:=Pwd
  End If
  If .Bookmarks.Exists(BmkNm) Then
    Set BmkRng = .Bookmarks(BmkNm).Range
    If .FormFields("Dropdown1").Result = "1000" Then
      .FormFields.Add BmkRng, wdFieldFormTextInput
    Else
      BmkRng.Delete
    End If
    .Bookmarks.Add BmkNm, BmkRng
  Else
    MsgBox "Bookmark: " & BmkNm & " not found."
  End If
  .Protect Type:=Prot, Password:=Pwd, NoReset:=True
End With
Set BmkRng = Nothing
End Sub