Deleting a message
See chat.delete documentation.
Find channel ID
In the Slack desktop app, right-click on the channel and select copy link. The last part of the URL is the ID.
In the browser version, navigate to the channel and check the URL. The last part of the URL is the ID.
Find message timestamp
In the Slack desktop app, right-click on the message timestamp and select copy link.
The last part of the URL is the timestamp and the last 6 numbers must be preceeded by a [ . ].
e.g.: 1612392726003500 -> 1612392726.003500