I think you can use NSURLSessionDownloadTask to send a POST.
IMO, download task doesn't mean it is used for download. it means the response of your POST request (json/xml) will be downloaded to a local file. Then you can open that file and parse it to get the request.
if you want you can even use a NSURLSessionDownloadTask to upload files to S3. and the s3 response will be 'downloaded' to a local file..
for more information, see this question in the apple developer forum https://devforums.apple.com/thread/210515?tstart=0