Add a string
#include <GuiComboBox.au3>
_GUICtrlComboBox_AddString ( $hWnd, $sText )
| $hWnd | Control ID/Handle to the control | 
| $sText | String to add | 
| Success: | the index of the new item. | 
| Failure: | -1. | 
_GUICtrlComboBox_DeleteString, _GUICtrlComboBox_InitStorage, _GUICtrlComboBox_InsertString, _GUICtrlComboBox_ResetContent
#include <DirConstants.au3>
#include <GuiComboBox.au3>
#include <GUIConstantsEx.au3>
Example()
Func Example()
    Local $idCombo
    ; Create GUI
    GUICreate("ComboBox Add String", 400, 296)
    $idCombo = GUICtrlCreateCombo("", 2, 2, 396, 296)
    GUISetState(@SW_SHOW)
    ; Add files
    _GUICtrlComboBox_AddDir($idCombo, "", $DDL_DRIVES, False)
    ; Add string
    _GUICtrlComboBox_AddString($idCombo, "This string has been added")
    ; Loop until the user exits.
    Do
    Until GUIGetMsg() = $GUI_EVENT_CLOSE
    GUIDelete()
EndFunc   ;==>Example