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This guide shows you how to use ngrok to receive Mailchimp webhooks on your localhost app. By integrating ngrok with Mailchimp, you can:
  • Develop and test Mailchimp webhooks locally without deploying to a public environment or setting up HTTPS.
  • Inspect and troubleshoot requests from Mailchimp in real time via the inspection UI and API.
  • Modify and replay Mailchimp webhook requests with a single click instead of reproducing events manually in your Mailchimp account.
  • Secure your app with Mailchimp webhook validation provided by ngrok. Invalid requests are blocked by ngrok before reaching your app.

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What you’ll need

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1. Start your app

For this tutorial, you can use the sample Node.js app on GitHub. To install the sample, run the following in a terminal: 
git clone https://github.com/ngrok/ngrok-webhook-nodejs-sample.git
cd ngrok-webhook-nodejs-sample
npm install
Then start the app: 
npm start
The app runs on port 3000 by default. You can confirm it’s running by visiting http://localhost:3000. The app logs request headers and body in the terminal and shows a message in the browser.

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2. Expose your app with ngrok

Once your app is running locally, you’re ready to put it online securely using ngrok.
The ngrok agent uses your authtoken to authenticate when you start a tunnel.
  • Start ngrok: 
    ngrok http 3000
  • Copy the URL ngrok displays. Your app is now exposed at that URL for use with Mailchimp.

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3. Configure Mailchimp to send webhooks

Mailchimp can send webhook requests to your app when events occur in your audience. To register for those events:
  • Sign in to Mailchimp.
  • Click Audience and then Audience dashboard.
  • Click Manage Audience and Settings.
  • On the Settings page, click Webhooks and then Create New Webhook.
  • On Create new webhook, enter your ngrok URL in Callback URL (for example, https://1a2b-3c4d-5e6f-7g8h-9i0j.ngrok.app) and click Save.
Confirm your localhost app receives the GET request and logs both headers and body in the terminal.

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Run webhooks with Mailchimp and ngrok

Mailchimp sends different request body contents depending on the audience and event. To trigger new calls from Mailchimp to your app:
  • Click Audience and All Contacts.
  • Click Add contacts and Add a subscriber.
  • On Add subscriber, enter a valid email in Email Address, check This person gave me permission to email them, and click Subscribe.
If the email is not on your contact list, you may need to import the contact first.
Confirm your localhost app receives the event and logs both headers and body in the terminal.

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Inspecting requests

ngrok’s Traffic Inspector captures all requests made through your ngrok endpoint to your localhost app. Select any request to view detailed information about both the request and response.
To avoid exposing secrets, accounts only collect traffic metadata by default. You must enable full capture in the Observability section of your account settings to capture complete request and response data.
Use the traffic inspector to:
  • Validate webhook payloads and response data
  • Debug request headers, methods, and status codes
  • Troubleshoot integration issues without adding logging to your app

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Replaying requests

Test your webhook handling code without triggering new events from your service using the Traffic Inspector’s replay feature:
  1. Send a test webhook from your service to generate traffic in your Traffic Inspector.
  2. Select the request you want to replay in the traffic inspector.
  3. Choose your replay option:
    • Click Replay to send the exact same request again
    • Select Replay with modifications to edit the request before sending
  4. (Optional) Modify the request: Edit any part of the original request, such as changing field values in the request body.
  5. Send the request by clicking Replay.
Your local application will receive the replayed request and log the data to the terminal.