A typical modern pocket door has strips of 3/4” thick struts running horizontally. You’ll be able to see these through the gap when the pocket door is closedall the way in. Hit these strips with a 1-1/8” screw (keep in mind the hook keeps the screw a little proud of the drywall) and you should be fine. (Test the movement of the door after you’ve driven the screws to make sure, and be prepared to nip the tips, but I doubt that’ll be an issue.)