Run a function on custom button click in WooCommerce admin order list (HPOS)

Based on "Add a button on top of admin orders list in woocommerce" answer code, I was able to add a custom button on woocommerce admin orders list.

Here is that code (lightly customized):

add_action( 'manage_posts_extra_tablenav', 'admin_order_list_top_bar_button', 20, 1 );
function admin_order_list_top_bar_button( $which ) {
    global $typenow;

    if ( 'shop_order' === $typenow && 'top' === $which ) {
        ?>
        <div class="alignleft actions custom">
            <button type="submit" name="custom_" style="height:32px;" class="button" value=""><?php
                echo __( 'Import Couriers', 'woocommerce' ); ?></button>
        </div>
        <?php
    }
}

Now I need to run a the following function when this custom button is clicked:

function update_shipping_couriers_meta_field() {
    $dir = __DIR__;
    $couriers = file( $dir . '/import-couriers.csv', FILE_IGNORE_NEW_LINES | FILE_SKIP_EMPTY_LINES );
    $count = count(couriers);
    $i = 1;

    do {
        if ( !empty( $couriers ) ) {
            foreach ( $couriers as $a ) {
                if ( !empty( $a ) ) {
                    $rows = explode(';', $a);

                    $id = $rows[0];
                    $id = int($id);
                    $couriers = $rows[1];

                    update_post_meta( $id, '_shipping_couriers', $couriers );
                }
                $i++;
            }
        }
    } 
    while ( $i <= $count );
}

In practice, the function updates a "_shipping_couriers" custom field based on a specific order ID. The two values ​​are present in a csv file.

I've already tested it and it's working. I "just" have it run when I click on the button I created with the function above.

How can I run my function when the button is clicked?

Update for High Performance Order Storage (HPOS).

There are some missing things in your code and an error in your last function where count(couriers); need to be instead count($couriers);.

Try the following:

// Display an action button in admin order list header:
add_action( 'manage_posts_extra_tablenav', 'admin_order_list_top_bar_button', 20, 1 );
function admin_order_list_top_bar_button( $which ) {
    global $pagenow, $typenow;

    if ( 'shop_order' === $typenow && 'edit.php' === $pagenow && 'top' === $which ) {
        printf('<div class="alignleft actions import-courier">
        <button type="submit" name="import_courier" style="height:32px;" class="button" value="%s">%s</button>
        </div>', 'yes', esc_html__('Import Couriers', 'woocommerce') );
    }
}

// Trigger an action (or run some code) when the button is pressed:
add_action( 'restrict_manage_posts', 'admin_order_list_update_shop_orders_action' );
function admin_order_list_update_shop_orders_action() {
    global $pagenow, $typenow;

    if ( 'shop_order' === $typenow && 'edit.php' === $pagenow &&
    isset($_GET['import_courier']) && $_GET['import_courier'] === 'yes' ) {
        
        ## -------- The code to be trigered -------- ##
        
        update_shipping_couriers_meta_field();
        
        ## -------------- End of code -------------- ##
    }
}

// Utility function that will be triggered on button press
function update_shipping_couriers_meta_field() {
    $dir = __DIR__;
    $couriers = file( $dir . '/import-couriers.csv', FILE_IGNORE_NEW_LINES | FILE_SKIP_EMPTY_LINES );
    $count = count($couriers);
    $i = 1;
    
    do {
        if ( ! empty( $couriers ) ) {
            foreach ( $couriers as $a ) {
                if ( ! empty( $a ) ) {
                    $rows = explode(';', $a);
    
                    update_post_meta( intval($rows[0]), '_shipping_couriers', $rows[1] );
                }
                $i++;
            }
        }
    } 
    while ( $i <= $count );
}

To make is work when High Performance Order Storage (HPOS) is enabled, replace the first 2 functions with the following ones:

// Display an action button in admin order list header:
add_action( 'woocommerce_order_list_table_extra_tablenav', 'admin_hpos_wc_order_list_top_bar_button', 20, 2 ); 
function admin_hpos_wc_order_list_top_bar_button( $order_type, $which ) {
    if( 'shop_order' === $order_type && 'top' === $which ) {
        printf('<div class="alignright actions import-courier">
        <button type="submit" name="import_courier" style="height:32px;" class="button" value="%s">%s</button>
        </div>', 'yes', esc_html__('Import Couriers', 'woocommerce') );
    }
}

// Trigger an action (or run some code) when the button is pressed:
add_action( 'woocommerce_order_list_table_restrict_manage_orders', 'admin_hpos_wc_order_list_update_action', 20, 2 );
function admin_hpos_wc_order_list_update_action( $order_type, $which ) {

    if ( 'shop_order' === $order_type &&  'top' === $which &&
    isset($_GET['import_courier']) && $_GET['import_courier'] === 'yes' ) {
        
        ## -------- The code to be trigered -------- ##
        
        update_shipping_couriers_meta_field();
        
        ## -------------- End of code -------------- ##
    }
}

See: Filtering orders list in WooCommerce with HPOS

Code goes in functions.php file of your child theme (or in a plugin file).

Based on: Add a button on top of WooCommerce admin orders list (HPOS)