JJ Thomas

Hi all, Would really appreciate help on this. I have a highlife envoy and I just filled up hot tub after draining and I then tested for water hardness and it is very hard (I would say around 400ppm). My dealer supplied us with a vanishing act when we bought it but I am unsure if I use this after the hot tub has heated up or now and then I heat it up. Also would the salt and liquid chlorine then be added after this? Thanks in advance