Choose Integrations from the Mattermost sidebar menu, then Incoming Webhooks, and click the "Add Incoming Webhook" button. Enter the options you want, then save the new webhook. Copy the URL displayed on the next page, and then enter that URL into the Webhook URL field in the Honeybadger UI.
That's it! You can test the integration by clicking "Test". Otherwise, just save it and you're ready to go.