Box Collider

How do I add a touch input to a box collider, so when the user touches their phone during gameplay, a pop up window happens…

Theoretically you can just set the players touch as an object and just say OnTriggerEnter to check and then add a statement referencing the popup GUI. To get this to work you probably should research how to register where a player taps on a screen and see if maybe the player tapping could be under the same reference as using a mouse on a computer. Yet again this is theoretical I haven’t checked this myself. If you are doing this make sure to turn on IsTrigger on the box collider in the inspector.