functions.php

<?php
function pmgarman_custom_wc_thankyou_action( $order_id = 0 ) { ?>
  <script>
    var leadNumber = '<?php echo $order_id; ?>';
  </script>
<?php }
add_action( 'woocommerce_thankyou', 'pmgarman_custom_wc_thankyou_action' );