Try this :
#include <ButtonConstants.au3>
#include <EditConstants.au3>
#include <GUIConstantsEx.au3>
#include <StaticConstants.au3>
#include <WindowsConstants.au3>
#Region ### START Koda GUI section ###
$Form1_1 = GUICreate("Form1", 372, 218, 192, 124)
GUISetBkColor(0x008080)
$idUsername = GUICtrlCreateInput("Username", 35, 56, 305, 21, BitOR($GUI_SS_DEFAULT_INPUT,$ES_CENTER))
$idUseremail = GUICtrlCreateInput("email", 35, 84, 305, 21, BitOR($GUI_SS_DEFAULT_INPUT,$ES_CENTER,$ES_LOWERCASE,$ES_READONLY))
$idPassword = GUICtrlCreateInput("password", 35, 112, 305, 21, BitOR($GUI_SS_DEFAULT_INPUT,$ES_CENTER))
$Computer = GUICtrlCreateLabel("Computer Setup", 80, 8, 213, 34)
GUICtrlSetFont(-1, 18, 400, 0, "Showcard Gothic")
$Exit = GUICtrlCreateButton("Exit", 216, 168, 123, 25)
GUISetState(@SW_SHOW)
#EndRegion ### END Koda GUI section ###
While 1
$nMsg = GUIGetMsg()
Switch $nMsg
Case $GUI_EVENT_CLOSE
Exit
Case $idUsername
GUICtrlSetData($idUseremail, GUICtrlRead($idUsername) & "@email.com")
Case $Exit
Exit
EndSwitch
WEnd
Edit : @anutt217
Explanation :
GUICtrlCreateInput returns a controlID on success, not the data (in this case e.g. not the 'username') itself. To read or set the data you have to use GUICtrlRead or GUICtrlSetData.