Because this was the [[zava'ah]] of your [[Rebbe]]! If you don't do it he will not recognize you at the resurrection.
What is the symbolism in the blessing over the [[Kos shel Brakha]] after [[Seuda Shlishit]] [[Hitveadut]] ([[https://en.m.wikipedia.org/wiki/Tish_(Hasidic_celebration) Tish]]) for [[Havdalah]] on [[Motzei Shabbat]]?
Your Rebbe has decreed for you that this blessing is to be regarded as the toast of the [[Brit Chadasha]] for the sake of which he made [[Kiddush HaShem]]. Anyone who makes this toast is in the Brit Chadasha. The sages taught us that the blessing over the Kos for Havdalah is guaranteed to merit you a place in [[Olam HaBa]]. It is the substitute for Toda'ah offerings in the [[Beit HaMikdash]] according to the Brit Chadasha.
