The problem is that $username and $password are referencing labels, not your inputs, plus you're not using GUICtrlRead on the $username and $password, you're just passing the handles to the ADOpen command.
;your code
GUICtrlCreateInput("Username", 112, 24, 121, 21)
GUICtrlCreateInput("Password", 112, 64, 121, 21, BitOR($GUI_SS_DEFAULT_INPUT,$ES_PASSWORD))
$Username = GUICtrlCreateLabel("Username", 24, 24, 52, 17)
$Password = GUICtrlCreateLabel("Password", 24, 64, 50, 17)
How it should be written
$Username = GUICtrlCreateInput("Username", 112, 24, 121, 21)
$Password = GUICtrlCreateInput("Password", 112, 64, 121, 21, BitOR($GUI_SS_DEFAULT_INPUT,$ES_PASSWORD))
GUICtrlCreateLabel("Username", 24, 24, 52, 17)
GUICtrlCreateLabel("Password", 24, 64, 50, 17)
; more code
_ADOpen(GUICtrlRead($username), GUICtrlRead($Password))