Above pic is the Prindle forestay and ring setup, I'm guessing the P18-2 is same? What the pic does not show is the shorter tail with thimble that acts as a block, the halyard runs up through the ring, through the thimble on the shorter tail, and back down through the ring again. With the mast stepped and forestay connected, ring flops down, but when raising the jib the ring pivots up as the "S" hook passes through.
(center pic on right)
This sets up the "hooking" action of the "S" hook onto the ring as can be seen in the pic on the bottom right.
Disengaging the hook from the ring entails grabbing the halyard "up" line in one hand and the jib tack in the other hand and alternating pulling down on each untill the hook comes free, can take numerous of attempts.
What is very important is that when you connect the upper forestay (pigtail) to the mast hound (tang), the short pigtail must be closest to mast. Also, before raising the jib, make sure the opening on the "S" hook also faces the mast.
If your setup does not look the same as above, please post pic of what you do have. HTH