Ideally when the front legs are about to leave the ground. This takes lots of practice. Count down as Zoe said or get into a halfseat one stride before the fence while you learn and then with practice you will be able to feel when to do it.
Hi! This helped me when I was first learning to canter jumps. I count my strides to the jump 3, 2 , and exactly after 1, i position myself into halfseat. Hope this works <3