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When someone signs up on your Hiveku site, you want them in Mailchimp a second later. This guide gets newsletter signups flowing straight into your Mailchimp audience with tags, merge fields, and optional double opt-in.

Via Zapier

Zero-code, 5 minutes. Good for low-to-medium volume.

Via Mailchimp API

Direct API integration. More control over tags and fields.

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Step 1: Get Your Mailchimp API Credentials

1

Log in to Mailchimp

Go to mailchimp.com and sign in.
2

Create an API key

Navigate to Account > Extras > API keys. Click Create a key. Copy the key — it looks like xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx-us20.
3

Note your data center

The suffix after the dash is your data center. In xxxx-us20, the DC is us20. You’ll need it for the API URL.
4

Find your Audience ID

Go to Audience > All contacts > Settings > Audience name and defaults. The Audience ID is near the bottom — looks like a1b2c3d4e5.

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Step 2: Store Credentials in Hiveku

Add three environment variables:
  • MAILCHIMP_API_KEY — the full key including the -us20 suffix
  • MAILCHIMP_AUDIENCE_ID — your audience ID
  • MAILCHIMP_DC — just the data center (e.g., us20)
See Environment Variables.

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Step 3: Create a Subscribe Workflow

1

Create the workflow

Go to Workflows > New Workflow. Name it Newsletter → Mailchimp.
2

Add a webhook trigger

Wire this URL to your newsletter signup form.
3

Add an HTTP Request action

POST https://{{env.MAILCHIMP_DC}}.api.mailchimp.com/3.0/lists/{{env.MAILCHIMP_AUDIENCE_ID}}/members
Headers:
  Authorization: Bearer {{env.MAILCHIMP_API_KEY}}
  Content-Type: application/json
Body:
{
  "email_address": "{{trigger.email}}",
  "status": "subscribed",
  "merge_fields": {
    "FNAME": "{{trigger.name}}"
  },
  "tags": ["website-signup"]
}

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Double Opt-In

Double opt-in is the safest pattern for deliverability and required in many jurisdictions (Germany, Canada). Change "status": "subscribed" to "status": "pending". Mailchimp will send its own confirmation email, and the subscriber only moves to subscribed after they click the link.
Double opt-in typically drops your signup rate by 10-20% but delivers far more engaged subscribers. For most newsletters the tradeoff is worth it.

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Tags for Segmentation

Tags let you segment later. Use them liberally: 
"tags": ["website-signup", "blog-reader", "2026-q2"]
Common patterns:
  • Source: website-signup, webinar-attendee, conference-2026
  • Content interest: ai-topics, design-topics
  • Stage: lead, customer, churned
Mailchimp lets you build segments from any tag combination in the UI.

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Merge Fields

Mailchimp’s default merge fields are FNAME, LNAME, EMAIL. Custom ones you create appear with whatever tag you set (e.g., COMPANY, ROLE). 
"merge_fields": {
  "FNAME": "{{trigger.first_name}}",
  "LNAME": "{{trigger.last_name}}",
  "COMPANY": "{{trigger.company}}"
}
Find the exact merge tag names in Mailchimp under Audience > Settings > Audience fields and |MERGE| tags.

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Unsubscribe Sync

When users unsubscribe in Mailchimp, you probably want to update your own CRM.
1

Configure a Mailchimp webhook

In Mailchimp, go to Audience > Settings > Webhooks. Create a webhook pointing to a Hiveku workflow webhook URL.
2

Subscribe to unsubscribe events

Check the Unsubscribes event.
3

Handle the event in Hiveku

The webhook delivers a payload with the email. Update your CRM: UPDATE contacts SET marketing_opt_in = false WHERE email = '{{trigger.data.email}}'.
Always respect consent. If a visitor didn’t explicitly opt in (pre-checked boxes don’t count), don’t add them to marketing lists. GDPR and CAN-SPAM penalties are serious — a single complaint can have cascading consequences.

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Avoiding Sync Loops

When Mailchimp sends you an unsubscribe webhook, don’t let your sync workflow push the change back to Mailchimp. Flag events that originated from Mailchimp and skip those writes: 
IF {{trigger.source}} == 'mailchimp' THEN skip sync to Mailchimp

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Alternative: Use Zapier

If you’d rather not touch the API, Zapier has a Mailchimp integration out of the box. See Connect Zapier. Trigger: form submission. Action: “Add subscriber to list.” Five minutes, no code.

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Verify It Worked

  1. Submit your newsletter form with a test email
  2. Check Workflows > Runs — the HTTP call should return 200
  3. In Mailchimp, go to Audience and search for the email — the subscriber should appear with the correct tags and merge fields

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Troubleshooting

The data center in the URL is wrong. Your API key ends with -us20 (or similar) — that part goes into the URL as us20.api.mailchimp.com. If the key ends in -us1, use us1.api.mailchimp.com. A mismatched DC always returns 404.
Expected if the email is already in the audience. Mailchimp treats this as a 400 response with title: "Member Exists". Handle it gracefully — treat it as a successful subscribe (it is, from your perspective). Log it and move on.
You’re using the wrong tag names. Mailchimp uses all-caps merge tags: FNAME not first_name, LNAME not last_name. Check Audience > Settings > Audience fields for the exact tags, including any custom ones you’ve added.
User unsubscribes in Mailchimp → Mailchimp webhook fires → Hiveku workflow updates your CRM → your CRM sync pushes back to Mailchimp → Mailchimp re-unsubscribes them → webhook fires again. Add a source flag to skip the reverse sync when the event came from Mailchimp.
The API key is valid but doesn’t have access to the audience. Either the key was generated under a different Mailchimp account, or your Mailchimp user role doesn’t have list access. Check the account dropdown in Mailchimp — you might be logged into the wrong one.

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What’s Next?

Newsletter Signup Form

Add the form to your site if you haven’t yet

Email Suppressions

Keep your list clean across tools