A message was sent to a channel
{
"type": "message",
"subtype": "group_topic",
"ts": "1358877455.000010",
"user": "U2147483828",
"topic": "hello world",
"text": "<@U2147483828|cal> set the group topic: hello world"
}
A group_topic message is sent when the topic for a private group is changed.
When an event occurs, we will send an HTTP POST request to your Request URL. The outer payload is uniform across all Events API methods, and contains fields like team_id and event. Learn more