Task: Start a Whereby video call from Mattermost

n8n workflow JSON:
{"nodes":[{"name":"Webhook","type":"n8n-nodes-base.webhook","position":[590,400],"webhookId":"822cce61-ff5f-4cea-b8ba-1822651786e3","parameters":{"path":"822cce61-ff5f-4cea-b8ba-1822651786e3","options":{},"httpMethod":"POST"},"typeVersion":1},{"name":"Set","type":"n8n-nodes-base.set","position":[780,400],"parameters":{"values":{"string":[{"name":"amudhan","value":"n8n-rocks"}]},"options":{},"keepOnlySet":true},"typeVersion":1},{"name":"Mattermost","type":"n8n-nodes-base.mattermost","position":[990,400],"parameters":{"message":"=Join me in a video call:","channelId":"={{$node[\"Webhook\"].json[\"body\"][\"channel_id\"]}}","attachments":[{"title":"=https://whereby.com/{{$json[$node[\"Webhook\"].json[\"body\"][\"user_name\"]]}}","title_link":"=https://whereby.com/{{$json[$node[\"Webhook\"].json[\"body\"][\"user_name\"]]}}"}],"otherOptions":{}},"credentials":{"mattermostApi":"mm_creds"},"typeVersion":1}],"connections":{"Set":{"main":[[{"node":"Mattermost","type":"main","index":0}]]},"Webhook":{"main":[[{"node":"Set","type":"main","index":0}]]}}}