I think it really depends on how much (if anything) about your PTSD and symptoms you are willing to disclose.
If you are willing to share that you have PTSD - or if they already know this - your employer may be required to see whether they can accommodate your seating request (I'm not sure about legal employment stuff outside the UK)
If you don't want them to know you have PTSD but are prepared to say it makes you anxious/you are a bit phobic when it comes to sitting facing the wall, they may still do what they can to help.
If you don't want to bring any of this to their attention, I guess all you can do is ask because it would be your preference not to sit facing the wall. You might get lucky, you might not. Can't hurt to ask!
If you don't disclose and do end up sitting facing the wall, that will have a significant impact on you - I hated the seating/desk arrangement and the whole open plan office environment of my old job but it's only now that I'm out of it that I can fully appreciate just how stressful it was for me every day. So if you do end up in that position, you'll need to find some ways to help you manage that.
As
@Nikie suggests, a mirror might help. You could casually let people who are seated around you/who will need to approach you regularly that you're easily spooked so request that they announce themselves as they approach so they don't make you just (you could do this humorously/with a light touch). You may need to take regular breaks from your desk to recalibrate. Could you work in different places - meeting rooms, canteen, hot-desk at other work stations) so that you move around a bit during each day and don't spend the whole day sitting in the position that's anxiety-making for you? Or you could at that point ask a colleague with a desk that would work better for you to swap with you (though do this ASAP as once people are settled at 'their desk' they can get a bit territorial about it and might not then like the thought of changing.
Good luck - I hope you get a good outcome.