Grafana button http POST issue: failed to fetch data

Hi guys,

Here is a university student and new to Grafana. I have read about button plugin for Grafana but however I still not able to use it. What I want to achieve is to do a simple control to on and off led through http POST.
the figure shows my button settings.
I want to post a value of 3 in json object to the htttp, but its not able to connect with it and show a message “Failed to fetch”. May I know what is the possible reason for this happened and solution of it?

Thank you.