Have you tried it with cURL, from a command line?
You can inspect the idnode calls made by the web interface using Chrome Developer Tools. Right-click on a network call and you can even copy it as a cURL command and see how the data is structured.
Their request content type is "x-www-form-urlencoded" not JSON, so I don't think you need the brackets, colon, or quotes. This works for me:
/usr/bin/curl --silent --data 'uuid=1234567890abcdefghijk' 'http://user:pass@localhost:9981/tvheadend/api/idnode/load'