The function is going to add this new custom post status into the list of available order statuses within the WooCommerce “Orders” and “Edit Orders” pages so that we can actually use it. We want to pass in the current order statuses so that we can go through them and insert our order status into the list where we’d like it.

add_custom_completed_status_to_order_statuses.php

<?php 
add_filter('wc_order_statuses', 'add_custom_status_to_order_statuses');
function add_custom_status_to_order_statuses($order_statuses)
{     
    // Replace 'my-custom-status' with your custom order status slug
    // Replace 'My Custom Status' with your custom order status name
    $new_order_statuses = array();
    foreach ($order_statuses as $key => $status) {
        $new_order_statuses[$key] = $status;
        if ('wc-completed' === $key) {
            $new_order_statuses['wc-my-custom-status'] = __('My Custom Status', 'text-domain'); x
        }
    }
    return $new_order_statuses;
}