Please refer to an official Slack documentation in case of a struggle: https://docs.slack.dev/

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Step 1 - Create a Slack app
Tutorial - https://api.slack.com/apps?new_app
Select a name, choose a workspace to associate your app with (bear in mind you'll probably be posting lots of test messages, so you may want to create a channel for sandbox use), then click Create App. If you've already created an app, you can use that one.

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Step 2 - Enable incoming webhooks
You'll be redirected to the settings page for your new app (if you're using an existing app, you can load its settings via your app's management dashboard).
From here, select Incoming Webhooks, and toggle Activate Incoming Webhooks to on.

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Step 3 - Create an incoming webhook
Now that incoming webhooks are enabled, the settings page should refresh and some additional options will appear. One of those options is a functional button called Add New Webhook to Workspace — please click it. This button triggers a shortcut version of the installation flow for Slack apps, one that is completely self-contained so that you don't have to actually build any code to generate an incoming webhook URL. You'll be sent back to your app settings, where you should see a new entry under the Webhook URLs for Your Workspace section. Your webhook URL will look something like this:

 https://hooks.slack.com/services/T00000000/B00000000/XXXXXXXXXXXXXXXXXXXXXXXX 

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Go back to 123Cluster environment, toggle Slack url box and paste your generated webhook URL to complete the notifications setup for the Slack channel.