Javascript: am besten auf Seite von Kontaktformular oder Theme-Settings:

<script type="text/javascript">
document.addEventListener( 'wpcf7submit', function( event ) {
   dataLayer.push({'event': 'form_submit'});
}, false );
</script>

PHP Variante (in Theme Functions):

add_action('wp_footer', 'cf7_datenschicht_event'); 

function cf7_datenschicht_event(){
	?> 
<script type="text/javascript">
	document.addEventListener('wpcf7mailsent', function(event){
		dataLayer.push({
			event: 'form_submit'
		});
		
	}, false );
</script>
<?php
}