[contact-form-7 id="42" title="Test Form"]

add_action( 'wp_footer', 'mycustom_wp_footer' );

function mycustom_wp_footer() {
?>
    <script type="text/javascript">
        document.addEventListener( 'wpcf7mailsent', function( event ) {
            //ga( 'send', 'event', 'Contact Form', 'submit' );
            if ( '42' == event.detail.contactFormId ) { // Change 123 to the ID of the form
                jQuery('#modalSuccess').modal('show'); //this is the bootstrap modal popup id
            }
        }, false );
    </script>
<?php
}