If it's a Squarespace contact form submission that you want to track, you'll need the ID of the block, and then listen for the event 'contact-form-submit'
<script>
document.addEventListener('DOMContentLoaded', function() {
var yourForm = document.getElementById('block-IDxxxxxxxx');
if (yourForm) {
yourForm.addEventListener('contact-form-submit', function() {
window.lintrk('track', { conversion_id: XXXXXXXX });
});
}
</script>