You have to add it with the window open
this is an example
window = guiCreateWindow(0.33, 0.35, 0.34, 0.31, "Test", true)
VEHICLE_SETTINGS = false
bindKey( "x", "down", function()
if isPedInVehicle ( localPlayer ) and VEHICLE_SETTINGS == false then
guiSetVisible( window, true) showCursor( true )
VEHICLE_SETTINGS = true
else
guiSetVisible( window,false) showCursor( false )
VEHICLE_SETTINGS = false
end
end)