functions.php

/**
 * Add the meta _transaction_id in the search fields.
 *
 * @param array $fields
 * @return array
 */
function wc_custom_order_filter_fields( $fields ) {
    if ( ! in_array( '_transaction_id', $fields ) ) {
        array_push( $fields, '_transaction_id' );
    }
    return $fields;
}
add_filter( 'woocommerce_shop_order_search_fields', 'wc_custom_order_filter_fields' );