Add the slack-notify npm package from the Actions editor to send a Slack message when a new user registers for your application.

Create a Post User Registration action.

Go to api.slack.com create a new app, active Incoming Webhooks, and get your Slack Webhook URL.

Your webhook URL contains a secret. Don't share it online, including public Git repositories.

In the Action editor store the Slack Webhook URL as a secret and add the slack-notify npm package.

In the Actions editor enter the follow code:

const initSlackNotify = require('slack-notify')

exports.onExecutePostUserRegistration = async (event) => {
  const slack = initSlackNotify(event.secrets.SLACK_WEBHOOK_URL);
  const message = `New User: ${event.user.email}`

  slack.success({
    text: message
  })
};