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.

i´m a beginner with javascipt and need help . how can i call the json-rpc function ( getData ) to get
data from API and how can i parse it, to display the result an MM.
the result of the call should be an JSON-RPC string .

	getData : function(){
	
		var self = this;
		var json = "http://xxx.xxx.xxx.xxx:3777/api/";		
		var retry = true;


	      var HTTP = new XMLHttpRequest();
		    HTTP.open("POST",json,true);
		    var rpc = JSON.stringify({"jsonrpc":"2.0", "method":"GetValue", "params":[18697], "id":0});
		   HTTP.setRequestHeader("Content-type", "application/json");
		   HTTP.send(rpc);
		 
	 },

Thanks Thomas