Hold on to the pole as you step your outside foot onto the base of the pole. Place both hands on the pole at chest height. Push into the pole with your outside foot and pull with your arms as you lower your head and simultaneously lift your inside foot towards the ceiling. Keep your inside knee close to the pole to the extend your leg fully, hooking the inside heel over the top of the pole. Keep both legs straight.
You do not need to be able to do the box splits to get this move, but working on your flexibility can make it a lot easier. Try this move from the floor before trying higher up the pole.