MailChimp 4 WordPress GTM data layer push after form submission

wp_gtm_data_layer_mailchimp_4_wordpress.js

/**
 * Hooking into MailChimp for WordPress
 * MC4WP JS Form Events: https://mc4wp.com/kb/javascript-form-events/
 * Form success
 */
(function($) {
    $(document).ready(function() {
        mc4wp.forms.on('748.success', function(form) {
            //GTM Data Layer Code Goes Here
            window.dataLayer = window.dataLayer || [];
            window.dataLayer.push({
                'event' : 'formSubmissionSuccess',
                'formId' : 'newsletterForm'
            });
        });
    });
})(jQuery);

Ah hello! I hadn't realized I had a comment. This is far too late to be of use to you, I'm sure, but I usually put that code in its own js file. As far as I know, this code still works with the MailChimp 4 WordPress plugin.