Slack connected service type is available in the list when feature flag is enabled.
Before configuring connected service, app needs to be created in Slack. More information here.
After the app has been created in Slack, configure connected service using Client ID and Client Secret from the created app.
Save connected service > open it for edit again and navigate to Step 2.
Copy value of Callback URL.
In Slack, go to Oauth & Permissions tab > delete the redirect URL entry that has been created from the manifest file. Add new redirect URL link copied on previous step.
After saving redirect URL, go back to the connected service and open it for edit.
Navigate to Step 2 and click the Authorize URL link.
Click "Allow"
After successful configuration service is fully configured.