Rafael,
Here are the steps I took to integrate Zendesk and Mattermost:
Create a simple webhook on Mattermost.
On Zendesk's side, you'll have to create a new HTTPS TARGET (Admin > Extensions > Targets). The "method" has to be POST and the "content type" JSON.
Once you've created a taget, now you have to create a TRIGGER (Admin > Triggers) that send's a JSON to Mattermost everytime a ticket is updated. You'll have to setup de conditions and then the actions.
In actions choose "Notifications: Notify Target", and then choose the target you've just created. The secret here is how you setup the JSON, because Mattermost has specific rules for that.
Here's an example of how I did:
{
"username": "Zendesk",
"icon_url": "https://zen-marketing-documentation.s3.amazonaws.com/docs/en/branding-logo-zendesk.png",
"text":"#{{ticket.id}} - {{ticket.title}} was updated by {{current_user.first_name}} ({{ticket.status}})",
"attachments": [
{"text":" {{ticket.latest_comment}} "}
]}
Hope it works for you!