Transportation between Milan MXP and Lugano

Hi fellow travelers,
Can someone help me with a transportation question? I’m flying in to Milan Malpensa and hoping to catch a train to Lugano Switzerland. I have a STP but I know that won’t be useable until i cross the border into Switzerland. I believe that I need to purchase a ticket for the Italian leg of the trip, but I don’t know where the border crossing is so that I can determine the destination from the Airport to the border, then on to Lugano using my STP.

Thanks for the help.
~JoAnne