You can build a small GUI that prompts for it or use two Input Boxes.
Here's a GUI example:
#include <EditConstants.au3>
#include <GUIConstantsEx.au3>
#include <StaticConstants.au3>
Opt("GUIOnEventMode", 1)
$fLogin = GUICreate("Enter Credentials", 450, 200)
GUISetOnEvent($GUI_EVENT_CLOSE, "_Exit")
$lText = GUICtrlCreateLabel("Please enter username and password", 45, 16, 340, 28, $SS_CENTER)
$iCredsUser = GUICtrlCreateInput("", 120, 55, 265, 20)
$iCredsPass = GUICtrlCreateInput("", 120, 85, 265, 21, BitOR($GUI_SS_DEFAULT_INPUT, $ES_PASSWORD))
$lUser = GUICtrlCreateLabel("Username:", 10, 55, 95, 20, $SS_RIGHT)
$lPass = GUICtrlCreateLabel("Password:", 10, 85, 95, 20, $SS_RIGHT)
$bRun = GUICtrlCreateButton("Run", 16, 160, 100, 30)
$bCancel = GUICtrlCreateButton("Cancel", 317, 160, 100, 30)
GUICtrlSetOnEvent(-1, "_Exit")
GUISetState(@SW_SHOW)
While 1
Sleep(100)
WEnd
Func _Exit()
GUIDelete($fLogin)
Exit
EndFunc ;==>_Exit
Users are usually prompted for credentials when running something that requires administrative access anyhow, have you tried #requireadmin?