Task: Post a message to a channel on Mattermost

n8n workflow JSON:
{"nodes":[{"name":"On clicking 'execute'","type":"n8n-nodes-base.manualTrigger","position":[250,300],"parameters":{},"typeVersion":1},{"name":"Mattermost","type":"n8n-nodes-base.mattermost","position":[450,300],"parameters":{"channelId":"7og48pm3tp87bxibq6dgrjpcue","attachments":[],"otherOptions":{}},"credentials":{"mattermostApi":"mattermost access"},"typeVersion":1}],"connections":{"On clicking 'execute'":{"main":[[{"node":"Mattermost","type":"main","index":0}]]}}}