Add 'extra' metadata to a SearchWP Entry

searchwp-customizations.php

<?php

// Customize a SearchWP Entry before it is indexed.
add_filter( 'searchwp\entry\data', function( $data, $entry ) {
  // Use this key when adding a Custom Field Attribute.
  $extra_meta_key = 'my_extra_meta';
  
  $value = 'This is extra data to index as extra meta';
  
  // SearchWP's Post Meta Attribute expects that data is made of Tokens.
  $data['meta'][ $extra_meta_key ] = new \SearchWP\Tokens( $value );
  
  return $data;
}, 20, 2 );