Something like that in your functions.php
add_action( 'wp_footer', 'mycustom_wp_footer' );
function mycustom_wp_footer() {
?>
<script>
document.addEventListener( 'wpcf7mailsent', function( event ) {
if ( 'FORM-ID-HERE' == event.detail.contactFormId ) {
var liPixelSrc = "https://dc.ads.linkedin.com/collect/?pid=XXXXX&conversionId=XXXXXX&fmt=gif";
var liPixel = new Image();
liPixel.src = liPixelSrc;
return false;
};
}, false );
</script>
<?php
}
