At this time, webhook notifications cannot be sent from MailChannels to Slack.
Slack requires the JSON payload to be sent in a specific format that is incompatible with the MailChannels format. As such, any requests to post to Slack gets rejected.
A workaround is to use Zapier to consume the webhook and send a message to Slack.
If you have product feedback, please use the link https://info.mailchannels.com/product-feedback