Thanks. But i have an other problem.
The JSON request works per curl on console, but in js : "net::ERR_EMPTY_RESPONSE "

Curl : curl -i -X POST -H "Content-Type: application/json" -d "{\"jsonrpc\": \"2.0\", \"id\": \"0\", \"method\": \"GetValue\", \"params\": [18697]}" http://xxx.xxx.x.xx:3777/api/

but in the MM-JS-Script don´t work. Looks like the json parameter don´t work.