How to return value (Webhook)

I'm trying to use api.ai

link

From what I understand, I have to return a JSON in the format:

"fulfillment": {
     "speech": "Today in Boston: Fair, the temperature is 37 F",
     "source": "apiai-weather-webhook-sample",
     "displayText": "Today in Boston: Fair, the temperature is 37 F"
}

But how can I send this message through Webhook?