<?php

// Add additional content to a specific Custom Field during indexing in SearchWP.
add_filter( 'searchwp\source\post\attributes\meta\my_custom_field_key', function( $meta_value, $args ) {
  // Add 'coffee' to Custom Field value to make it searchable.
  return $meta_value[0] . ' coffee';
}, 20, 2 );