Thanks @Ole, that seems to work if I submit my JSON in the body as raw encoded JSON. But to my surprise there is no content/JSON delivered back to me. I get a 200 response, but it does not have any content other than {} while the rcd spits out lots of lines of what it actually checks. I am puzzled, what is the use of that option if I do not get a result back?
BTW, I fire up the remote control daemon with:
rclone rcd /home/pi/data --rc-user=bot --rc-pass=secret --rc-addr=0.0.0.0:8888 --rc-allow-origin=0.0.0.0 --rc-web-gui-no-open-browser --log-level=DEBUG