Thanks for the quick reply. I’m making the same request successfully using the macOS Rested application, which allows the same parameterization of the POST request:
http_post('https://...', atom(Body), Reply, [header('Content-Type','application/x-www-form-urlencoded')]).
Body is the same payload I use in Rested. I was wondering about HTTPS support due to:
ERROR:  throw(error(existence_error(url,'https://...'),context(_952,...)))
The URL is correct, no typos there. I will continue debugging. Thanks for the tips.