Task: Webhooks with Mattermost

n8n workflow JSON:
{"id":"13","name":"Mattermost Webhook","nodes":[{"name":"Webhook","type":"n8n-nodes-base.webhook","position":[340,200],"parameters":{"path":"webhook","options":{},"httpMethod":"POST"},"typeVersion":1},{"name":"HTTP Request","type":"n8n-nodes-base.httpRequest","position":[570,200],"parameters":{"url":"https://www.thecocktaildb.com/api/json/v1/1/random.php","options":{}},"typeVersion":1},{"name":"Mattermost","type":"n8n-nodes-base.mattermost","position":[770,200],"parameters":{"message":"=Why not try {{$node[\"HTTP Request\"].json[\"drinks\"][0][\"strDrink\"]}}?\n{{$node[\"HTTP Request\"].json[\"drinks\"][0][\"strInstructions\"]}} Serve in {{$node[\"HTTP Request\"].json[\"drinks\"][0][\"strGlass\"]}}.","channelId":"={{$node[\"Webhook\"].json[\"body\"][\"channel_id\"]}}","attachments":[{"image_url":"={{$node[\"HTTP Request\"].json[\"drinks\"][0][\"strDrinkThumb\"]}}"}],"otherOptions":{}},"credentials":{"mattermostApi":"Mattermost"},"typeVersion":1}],"active":true,"settings":{},"connections":{"Webhook":{"main":[[{"node":"HTTP Request","type":"main","index":0}]]},"HTTP Request":{"main":[[{"node":"Mattermost","type":"main","index":0}]]}}}