> ## Documentation Index
> Fetch the complete documentation index at: https://docs.mailkick.app/llms.txt
> Use this file to discover all available pages before exploring further.

# Connect Customer.io

> Sync Mailkick emails to a Customer.io journey action or newsletter template

This integration lets you push your Mailkick email HTML into Customer.io **journey actions** or **newsletter templates**.

## Prerequisites

* You must be a **workspace admin**.
* You need a **Customer.io App API Key**.
* You must know your Customer.io **region**: **US** or **EU**.

## Step 1 — Find your Customer.io API key

In Customer.io, generate/copy an **App API Key** from your workspace settings (API keys section).

<Tip>
  If your workspace is on the EU region, make sure you also know you’re on <strong>EU</strong> (Mailkick needs it to use the correct API base URL).
</Tip>

## Step 2 — Add Customer.io in Mailkick

1. Open **Settings → Integrations**
2. Select **Customer.io**
3. Paste your **API key**
4. Select **Region** (US/EU)
5. (Optional) Fill **Site ID**
6. Click **Save changes**

<img style={{ borderRadius: '0.75rem' }} src="https://mintlify.s3.us-west-1.amazonaws.com/mailkick/app/doc/integration/integration_customer.io.jpg" alt="Mailkick Customer.io integration settings" />

## Step 2.1 — Set a “Sending tool link” on the email

Customer.io sync requires a link that identifies what you want to update.

1. Open your email in the editor
2. Open **Settings** (email settings)
3. Fill **Sending tool link**

Supported link formats:

* **Journey action**:
  * `.../workspaces/<workspaceId>/journeys/.../actions/<actionId>`
* **Newsletter template**:
  * `.../journeys/composer/newsletters/<newsletterId>/templates/<templateId>`

If the link is invalid, Mailkick will block the sync with a validation error.

## Step 3 — Sync an email

1. Click **Export**
2. Click **Sync to Customer.io**

If nothing changed since the last sync, Mailkick will skip the API call (“Already up to date”).

## Common issues

* **“Enter the Customer.io Sending tool link before syncing”**: the email is missing the link.
* **“Region must be us or eu”**: set the region in Settings → Integrations → Customer.io.