Here is a hacky way:
#include <WinAPISysWin.au3>
#include <GuiEdit.au3>
HotKeySet('{TAB}', 'Tab')
$hGUI = GUICreate('Test')
$cInput1 = GUICtrlCreateInput('test', 100, 100, 150, 25)
$hInput1 = GUICtrlGetHandle($cInput1)
$cInput2 = GUICtrlCreateInput('', 100, 200, 150, 25)
$cInput3 = GUICtrlCreateInput('', 100, 300, 150, 25)
GUISetState()
Do
Until GUIGetMsg() = -3
Func Tab()
HotKeySet('{TAB}')
If _WinAPI_GetFocus() = $hInput1 Then
_GUICtrlEdit_ReplaceSel($hInput1, @TAB)
Else
Send('{TAB}')
EndIf
HotKeySet('{TAB}', 'Tab')
EndFunc