That is the command I was using. I just tried it again (both by hitting 'execute' and by pressing the button). Neither worked. Here's a screenshot of the flic app (second picture shows the rest of the command used). I can confirm that cURL with this command does turn my receiver OFF.
https://bayimg.com/paNadaAgO
http://bayimg.com/PaNaeAagO
Are there any logs to debug or troubleshoot, what else can I try?