Securing rigs and hooks on tip-ups, especially classic “underwater” styles like HT’s Polar, can be challenging. While no method is perfect, I’ve found stretching a heavy duty rubber band around the frame opposite the spool does the trick.
Provided the overall length of your leader material is shorter than the length of the frame, simply grasp the braided Dacron tip-up line backing just above your leader, slip a couple wraps around the spool handle, then pull your leader tight by extending your hook to the rubber band.
As a bonus, this practice also keeps your leader material straight and prevents wire leaders from kinking.