Async Recursive Delete with cURL Without Removing Top-Level Directory

Question

How can I perform an asynchronous, recursive deletion of a directory's contents without deleting the top-level directory itself using the DELETE cURL command?

# Current cURL DELETE command
curl -v -H 'Content-Type: application/octet-stream' \
  -X DELETE \
  "https://${ENDPOINT}/webhdfs/v1/$DST?op=DELETE&recursive=true"

# Attempted asynchronous deletion with 'async=true', but not working.
# Want to keep the top-level directory 'P1' intact while deleting its contents.