Before following this guide please make sure you configure the “Custom” integration. At the very least, a Mailchimp audience to subscribe to should be selected.
You can then add a “sign-up to our newsletter” checkbox to any third-party form you like by including the following HTML in the form.
<label> <input type="checkbox" name="mc4wp-subscribe" value="1" /> Sign-up to our newsletter. </label>
Should you want to include a hidden field that will tell the plugin to subscribe people, use the following.
<input type="hidden" name="mc4wp-subscribe" value="1" />
Sending additional fields
By default, Mailchimp for WordPress will automatically extract an email and name field out of the submitted form values. If you want to send additional data to Mailchimp, you can prefix the name attribute with
For example, if you want to send an additional field called WEBSITE to Mailchimp, use the following HTML.
<label>Website:</label><br /> <input type="url" name="mc4wp-WEBSITE" placeholder="Your website.." />
This way you can add a sign-up checkbox to any form you like, provided you can add custom HTML to the forms.