This should be easy.
Remove the window frame and pull back the top of the door panel about an inch. You should see two raised humps in the door metal with holes. shove nails through the door panel to mark each hole. Use 1/8"x 3/4" oval-head sheet metal screws to attach the robe. Sometimes the holes have been enlarged over time requiring a wider screw. If painted I use a dish washer/cup between the screw bottom and the rope end piece to avoid cracking the paint.