Made it work whit this code.
Thanks
When.WebServiceCallReceived("HomeAutomation.NestLink/Update/", (jsonData) =>
{
    dynamic data = JsonConvert.DeserializeObject(((String)jsonData).Substring(31));
    // use the data object....
    MigService.Log.Debug(data.devices.thermostats);
    return "{ \"ResponseValue\" : \"OK\"}"
}); 
and here is my node.js part to send the data to Home genie
   var http = require('http');
   var options = {
     hostname: 'hg.erxinfo.ca',
     port: 8080,
     path: '/api/HomeAutomation.NestLink/Update/',
     method: 'POST',
     headers: {
              'Content-Type': 'application/json',
          },
     auth: 'xxx:xxx',
   };
   var hreq = http.request(options, (hres) => {
     hres.setEncoding('utf8');
     hres.on('data', (d) => {
            console.log(d);
     });
   });
   hreq.on('error', (e) => {
     console.error(e);
   });
   hreq.write(JSON.stringify(data));
   hreq.end();
Still need some work on the Node.js module....  and to be able to read all the valid data inside Home genie