CODE EXAMPLE FOR PHP

Search WordPress with custom field

function me_search_query( $query ) {
  if ( $query->is_search ) {
    $meta_query_args = array(
      array(
        'key' => 'your_key',
        'value' => $query->query_vars['s'],
        'compare' => 'LIKE',
      ),
    );
    $query->set('meta_query', $meta_query_args);
    add_filter( 'get_meta_sql', 'me_replace_and_with_or' );
  };
}

function me_replace_and_with_or( $sql ) {
    if ( 1 === strpos( $sql['where'], 'AND' ) ) {
        $sql['where'] = substr( $sql['where'], 4 );
        $sql['where'] = ' OR ' . $sql['where'];
    }

    //make sure that this filter will fire only once for the meta query
    remove_filter( 'get_meta_sql', 'me_replace_and_with_or' );
    return $sql;
}

add_filter( 'pre_get_posts', 'me_search_query');