Not useful in a test situation, but an exercise at home to help the horse to follow the connection down is to alternate stretch and picking back up. Somewhere around 6-10 strides each doing 2-3 steps at the end of that stretch or sit. Encourage stretch for the 6-10 without concerning yourself with how far the horse goes. Then gently shorten the reins and bring the horse into a more upright balance, asking the hind legs to step under the body. Repeat a bunch of times.
How much sit/upright posture you ask for depends on your horse’s current training and that day’s conditions. You’re not trying to get the maximum sit or stretch, just to get the response to the aids. As the exercise repeats the horse will stretch further and sit more. They might need more than 6-10 strides at first, perhaps half a 20m circle. That’s okay too. As they repeat the exercise and understand what you’re asking they’ll respond more quickly without tension.
As the horse stretches more and sits more you can do a few more steps at each end of the stretch/sit without asking for more before going back to the other extreme.