We are trying to send alerts to specific Slack channel via Webhook. I am seeing errors in both the legacy Webhook plugin, as well as with a Custom Integration.

Sentry’s Webhooks are designed to be received by your own code first, allowing you to transform the data before forwarding it to another service in the required format. Because of this and because Slack is expecting a specific type of payload, using the Slack webhook directly is not supported. You might be able to get this working by handling the Webhook payload yourself and then sending it to Slack. Otherwise, you should use our first-party Slack integration instead.