Workflow Setup: External HTTP Request With Callback Triggering a Slack Message in N8N

Here is the step-by-step documentation for setting up the N8N workflow based on your screenshots. This setup demonstrates an interaction between a trigger button, an external HTTP POST request with a callback URL, and a Slack message based on a webhook response.

Overview

This workflow allows you to:

1. Trigger a workflow manually.
2. Send an HTTP request to an external server with a callback_url.
3. Wait for the external system to call the callback URL.
4. When the callback is triggered, send a message to a Slack channel with the response.

1. Manual Trigger: Execute workflow

• Node Type: Trigger → Manual Trigger
• Purpose: Allows you to manually initiate the workflow in N8N for testing or production use.

2. HTTP Request to External Server

• Node Type: HTTP Request

• Method: POST

• URL:

  https://webhook-custom-dev-1080791310029.europe-west1.run.app/custom/v1/webhook
  

• Authentication:

  • Type: Generic Credential Type
  • Auth Method: Bearer Auth
  • Credential: Bearer Auth account (This is your Watermelon API key)

• Send Body: Enabled

• Body Content Type: JSON

• JSON Payload:

{
  "channel_name": "chatwidget",
  "channel_id": "",
  "text": "",
  "callback_url": ""
}

🔁

This sends the message to an external server, telling it where to POST the callback response once it has processed the request.

3. Webhook Listener

• Node Type: Trigger → Webhook

• Method: POST

• Path: 37343446-975a-411e-a05a-9b3281bff868

• Webhook URL:

  https://username.app.n8n.cloud/webhook/37343446-975a-411e-a05a-9b3281bff868
  

• Authentication: None

• Response Mode: Immediately

📥

This node listens for the external service to call back with the result.

4. Send Slack Message

• Node Type: Slack → Message
• Operation: Send
• Channel: gcp-errors (select from list)
• Message Type: Simple Text Message
• Message Text:

This is the agent response from N8N after the webhook was triggered {{ $json.body.message }}

📤

This sends a message to the specified Slack channel using the content from the webhook callback.

✅ Final Flow Summary

🧪 Testing Instructions

1. Click “Execute Workflow” to initiate the flow.
2. Wait for the external server to call the callback_url.
3. Observe the response message being sent to the gcp-errors Slack channel.

✅ Key Notes

• Ensure the callback_url in the HTTP request matches the Webhook node's Production/Test URL.
• Your external service must POST to the callback URL after handling the initial request.
• The expected response payload should include a message field for the Slack node to render it.

Let me know if you want a variant with conditional logic, multiple agents, or error handling.