This webhook integration for Trello is the recommended way to integrate with Trello, and should support all the features of the legacy Trello cron-based integration.

First, create the stream you'd like to use for Trello notifications, and subscribe all interested parties to this stream. We recommend the name trello.

Your webhook URL is:
{{ external_api_uri_subdomain }}/v1/external/trello?api_key=abcdefgh&stream=trello
where api_key is the API key of your Zulip bot, and stream is the stream name you want the notifications sent to.

Before you create a webhook, you'll need to follow the steps below to get an APPLICATION_KEY, and a UserToken, and to fetch the board's idModel.

To generate the APPLICATION_KEY, open this URL in your web browser:
https://trello.com/1/appkey/generate.

To generate a read access token, fill in and open this URL in the browser while logged into your Trello account: https://trello.com/1/authorize?key=<APPLICATION_KEY>&name=Issue+Manager&expiration=never&response_type=token&scope=read
You will receive your UserToken. Note it.

Within the the board URL, you can find the TRELLO_BOARD_SHORT_ID. The Trello URL format is:
https://trello.com/b/TRELLO_BOARD_SHORT_ID/boardName.

Now you have the APPLICATION_KEY, UserToken and TRELLO_BOARD_SHORT_ID.
Construct this URL and open it in your web browser:
https://api.trello.com/1/board/<TRELLO_BOARD_SHORT_ID>?key=<APPLICATION_KEY>&token=<UserToken>
You'll receive some JSON. Within that, find the id value. That's your idModel; note it.

Now you have the ingredients to create your webhook. Send a POST request to this URL with this data: https://api.trello.com/1/tokens/<UserToken>/webhooks/?key=<APPLICATION_KEY>

{
    "description": "Webhook for Zulip integration",
    "callbackURL": "<URL_TO_ZULIP_WEBHOOK_FROM_SECOND_STEP>",
    "idModel": "<ID_MODEL>",
}

You can use curl to do this:

curl 'https://api.trello.com/1/tokens/<UserToken>/webhooks/?key=<APPLICATION_KEY>'
-H 'Content-Type: application/json' -H 'Accept: application/json'
--data-binary $'{\n  "description": "Webhook for Zulip integration",\n  "callbackURL": "<URL_TO_ZULIP_WEBHOOK_FROM_SECOND_STEP>",\n  "idModel": "<ID_MODEL>"\n}'
--compressed

The response from Trello should look like:

{
    "id": "<WEBHOOK_ID>",
    "description": "Webhook for Zulip integration",
    "idModel": "<ID_MODEL>",
    "callbackURL": "<URL_TO_ZULIP_WEBHOOK_FROM_SECOND_STEP>",
    "active": true
}

Congratulations! You've created a webhook and your integration is live.

When you make changes in on this board in Trello, you will receive Zulip notifications like this: