API.AIのFulfillmentにIFTTTのWebhooksサービスを使う ref: http://qiita.com/mayfair/items/d6440cec8104bc72c6c7

API.AIが発行するwebhookの例（ドキュメントより抜粋）

POST https://my-service.com/action

Headers:
Content-type: application/json

POST body:
{
    "lang": "en", 
    "status": {
        "errorType": "success", 
        "code": 200
    }, 
    "timestamp": "2017-02-09T16:06:01.908Z", 
    "sessionId": "1486656220806", 
    "result": {
        ...
        "action": "greetings", 

app.js

'use strict';

const express = require('express');
const request = require('request');
const bodyParser = require('body-parser');

const app = express();
app.use(bodyParser.json());

app.post('/', (req, res) => {
  // API.AIからのwebhookのヘッダに記載されたkeyをIFTTTのkeyとして使用
  const KEY = req.headers['key'];

  // API.AIからのwebhookのresult/actionをIFTTTのイベントのvalue1として使用
  const ACTION = req.body['result']['action'];

  // IFTTTのイベント名は「api_ai」で固定
  var options = {
    uri: 'https://maker.ifttt.com/trigger/api_ai/with/key/' + KEY,
    form: {
      value1: ACTION,
      value2: '',
      value3: ''
    },
    json: true
  };

  request.post(options, (error, response, body) => {
    let responseToApiAi;

    if (!error && response.statusCode === 200) {
      console.log(body);

      responseToApiAi = {
        'speech': 'OK!',
        'displayText': 'OK!'
      };
    } else {
      console.log('error: ' + response.statusCode);

      responseToApiAi = {
        'speech': 'ERROR!',
        'displayText': 'ERROR: ' + response.statusCode
      };
    }

    res.json(responseToApiAi);
  });
});

const PORT = process.env.PORT || 8080;
app.listen(PORT, () => {
  console.log(`App listening on port ${PORT}`);
});

IFTTTのWebhooksサービスが受け取るwebhookの例

POST/GET https://maker.ifttt.com/trigger/{event}/with/key/*********************

Headers:
Content-Type: application/json

POST body:
{
    "value1":"value1"
}