Create a Slack integration in the following steps
Create a Slack app (if you don't have one already)
Go to Slack Apps and create an app or select an existing one.
Enable incoming webhooks
Go to Incoming Webhooks in your App Management Board and activate this feature.
Create an incoming webhook
You can create a new webhook in the same menu
Click on Add New Webhook to Workspace
Choose your workspace and channel
After creating the webhook, copy the webhook url into the URL field of the Gateway Manager.
https://api.slack.com/messaging/webhooks
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