During the main story, you'll get a pizza delivery request from a prepper named Peter Englert, be sure you complete that request.
Eventually (I think after episode 9 maybe), you'll get another pizza delivery request from the same guy, starting from South Knot City. When you get to the prepper's base you'll notice that the door is open, so you can simply walk past the delivery terminal and enter the room behind it.
Be aware that the pizza cannot be simply stashed in the backpack, you have to keep it horizontal so if you want to use the back pack you have to carefully place packages so you can place the pizza on top. Otherwise you can always use an hoverboard.