You can’t, at least not through Add From Server.
WordPress automatically creates thumbnails for all uploaded images, if you wish to disable those thumbnails you can probably find a plugin to do it – but you’ll then have no thumbnails unless you’re using something like Photon (part of Jetpack) which does image resizing on-the-fly. I’m not aware of any plugins which would only be able to apply that to Add From Server though.
I was able to prevent them from being generated from the functions file but wordpress is still generating 768×1024 files and I’m not sure how to remove that.
