Before following this guide please make sure you correctly setup your form integration settings.
You can 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" />
The plugin will automatically extract an email and name field out of the form. Should you want to send additional data to Mailchimp, you can prefix the name attribute with mc4wp-. For example, if you want to send an additional field called WEBSITE to Mailchimp, use the following HTML.
<label>Website:</label> <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.