How to Send Form Submissions to Kit (ConvertKit) in 2026

Why route Kit signups through your own form

Kit (formerly ConvertKit) is a great email platform, and its embeddable forms are fine for a quick newsletter box. But the moment you want a form that's fully on-brand, fast-loading, spam-protected, and logged in a dashboard you control, the hosted embed gets in the way.

Own the HTML and route it through splitforms: you get a custom form, an AI spam classifier, email notifications, and a submission dashboard — while Kit stays your email home. A small proxy turns each signed webhook into a Kit subscribe call, with the tags and sequence you want.

Step 1 — Get your Kit API credentials and target IDs

In Kit, open Settings → Advanced → API and copy your API key (and API secret if you use endpoints that require it). Decide where new subscribers should land — a specific form or sequence — and copy its numeric ID from the URL when editing it. Optionally grab tag IDs from Subscribers → Tags.

Store the API key, form/sequence ID, and tag IDs as environment variables on your proxy host. Never put them in the browser.

Step 2 — The splitforms → Kit proxy

splitforms fires a signed webhook to your endpoint; the proxy holds the API key and subscribes the contact to your chosen form. Cloudflare Worker version:

export default {
  async fetch(req, env) {
    if (req.method !== "POST") return new Response("Method Not Allowed", { status: 405 });

    // 1. (Recommended) verify the splitforms HMAC signature header here.

    const submission = await req.json();
    const f = submission.data ?? submission;
    if (!f.email) return new Response("no email", { status: 400 });

    // 2. Subscribe to a Kit form (v3 endpoint; still served at api.convertkit.com)
    const res = await fetch(
      `https://api.convertkit.com/v3/forms/${env.KIT_FORM_ID}/subscribe`,
      {
        method: "POST",
        headers: { "Content-Type": "application/json" },
        body: JSON.stringify({
          api_key: env.KIT_API_KEY,
          email: f.email,
          first_name: (f.name ?? "").split(" ")[0],
          fields: { last_message: f.message ?? "", source: f.source_page ?? "website" },
          tags: env.KIT_TAG_IDS ? env.KIT_TAG_IDS.split(",").map(Number) : undefined,
        }),
      }
    );

    if (!res.ok) return new Response(await res.text(), { status: 502 });
    return new Response("ok", { status: 200 });
  },
};

Deploy the Worker and paste its URL into your form's webhook settings in the splitforms dashboard. To subscribe to a sequence instead of a form, swap the URL to the sequences subscribe endpoint. Always verify the webhook signature — see send form data to a webhook.

Step 3 — The form

Your public form only talks to splitforms with the public access key. Swap YOUR_ACCESS_KEY for the one from your free account.

<form action="https://splitforms.com/api/submit" method="POST">
  <input type="hidden" name="access_key" value="YOUR_ACCESS_KEY" />
  <input name="name" placeholder="First name" />
  <input name="email" type="email" required placeholder="Email" />

  <label><input type="checkbox" name="consent" required /> Subscribe me to the newsletter</label>

  <!-- honeypot -->
  <input type="checkbox" name="botcheck" style="display:none" tabindex="-1" autocomplete="off" />
  <button type="submit">Subscribe</button>
</form>

What to do next

• Other email tools: Brevo · ActiveCampaign · Mailchimp
• Webhook fundamentals: send form data to a webhook
• Consent + privacy: GDPR-compliant submissions
• Ready to set it up: get a free access key

FAQ