Basic example to demonstrate how to encode json data and POST it to an arbitrary web service.

httpjsonencode.agent.nut

device.on("senddata", function(data) {
  // Set URL to your web service
  local url = "https://www.mywebservice/object";

  // Set Content-Type header to json
  local headers = { "Content-Type": "application/json" };

  // encode data and log
  local body = http.jsonencode(data);
  server.log(body);
  
  // send data to your web service
  http.post(url, headers, body).sendsync();
});

httpjsonencode.device.nut

// placeholder to ignore digital input change
function nullfunction() {}

// configure imp
hardware.pin1.configure(DIGITAL_IN, nullfunction);
hardware.pin2.configure(ANALOG_IN);


function loop() {
  // read values
  local pin1Value = hardware.pin1.read();
  local pin2Value = hardware.pin2.read();

  // create table and send to agent
  local data = { "pin1": pin1Value, "pin2": pin2Value }
  agent.send("senddata", data);

  // recursively call self after 15 seconds
  imp.wakeup(15.0, loop);
} 

// start the loop
loop();

There is a typo on line 13 of httpjsonencode.agent.nut: headerss should read headers

Also, line 6 should end in }; instead of );