You can use Javascript Events to track form interactions.

Use the following code if you are using GTM. Check here for other analytic integrations.

<script type="text/javascript">
mc4wp.forms.on('subscribed', function(form) {
   window.dataLayer = window.dataLayer || [];
   window.dataLayer.push({ 'event' : 'Sign-up', 'name' : + ' ID: ' + });

If you are using Google Tag Manager you need to create a Custom Event and Tag in your GTM control panel for it to work. In case of the code above the event name would be “Sign-up””.