Hi Xavier,
You can use the script for Slack to send the notification to Mattermost.
The only thing to do is to create a proper Mattermost incoming webhook and set the webhook in manager's ossec.conf like if we were using Slack
<integration>
<name>slack</name>
<hook_url>https://xxxxxxxxx.cloud.mattermost.com/hooks/xxxxxxxxxxxxxxxxxxxxxxxxxxx</hook_url>
<level>3</level>
<alert_format>json</alert_format>
</integration>
After restarting the manager, new alerts will appear in the Mattermost channel.
sudo systemctl restart wazuh-manager
Ref: https://documentation.wazuh.com/current/user-manual/manager/integration-with-external-apis.html
Let me know if this works for you.