"90S" will not take you anywhere as it is not a complete waypoint.
It would have to be in a lat/long format with the " S" first as in S90E127 .
I think I would choose S90E0 if I wanted to go to the pole. A waypoint behind the pole could do as well something like "Tierra del Fuego". If the south pole is chosen, the track must go straight down a meridian. If the flight path was a great circle or a constant heading path should be deductible by the ping arcs (hope that word is not banned yet).