Usage info
This method pins a message to a particular conversation or channel.
Both the channel and timestamp arguments are functionally required. The provided channel should be the ID of a public or private channel you want to pin to and the timestamp should be the ts value of a message within that conversation.
In the past, files and file comments could be pinned to a channel as well.
Response
After processing, a pin_added event is broadcast via the Events and RTM APIs.
Some objects cannot be pinned: channel join messages, files, and file comments. A not_pinnable error is thrown when we "no can do."