Example Form Link: https://leomeasure.store/book-appointment/
Listener Code: 

var form = document.querySelector('form.wpcf7-form');
form.addEventListener('submit', function() {
    var formData = new FormData(form);
    var inputs = {};
    formData.forEach(function(value, key) {
        inputs[key] = value; 
    });

    if(inputs['first-name'] && inputs['last-name'] && inputs['email-address'] && inputs['email-address'].includes('@') && inputs['subject']) {
        window.dataLayer.push({
            event: 'custom_form_submit',
            formInputs: inputs
        });
    }
});