Shot in the dark:
#include <ButtonConstants.au3>
#include <EditConstants.au3>
#include <GUIConstantsEx.au3>
#include <StaticConstants.au3>
#include <WindowsConstants.au3>
$Form1 = GUICreate("Form1", 382, 256)
$site = GUICtrlCreateInput("", 16, 48, 169, 21)
$Extension = GUICtrlCreateInput("", 200, 50, 161, 21)
$Button1 = GUICtrlCreateButton("Link create", 88, 176, 195, 33)
$Label1 = GUICtrlCreateLabel("Link:", 24, 88, 27, 17)
$Label2 = GUICtrlCreateLabel("Sample Link:http://autoitscript.com", 16, 24, 167, 20)
$Label3 = GUICtrlCreateLabel("Extension:AbFrT", 200, 24, 113, 17)
$Link = GUICtrlCreateInput("", 24, 112, 337, 21)
GUISetState(@SW_SHOW)
While 1
$nMsg = GUIGetMsg()
Switch $nMsg
Case $GUI_EVENT_CLOSE
Exit
Case $Button1
GUICtrlSetData($Link, GUICtrlRead($site) & GUICtrlRead($Extension))
EndSwitch
WEnd