Task: Send message on Mattermost when your n8n instance starts

n8n workflow JSON:
{"nodes":[{"name":"n8n Trigger","type":"n8n-nodes-base.n8nTrigger","position":[450,200],"parameters":{"events":["init"]},"typeVersion":1},{"name":"Mattermost","type":"n8n-nodes-base.mattermost","position":[650,200],"parameters":{"message":"=Your n8n instance started at {{$json[\"timestamp\"]}}","channelId":"toyi3uoycf8rirtm7d5jm15sso","attachments":[],"otherOptions":{}},"credentials":{"mattermostApi":"Mattermost Credentials"},"typeVersion":1}],"connections":{"n8n Trigger":{"main":[[{"node":"Mattermost","type":"main","index":0}]]}}}