How to add Custom Description field in Shipping methods (Backend)

add_action('woocommerce_init', 'shipping_instance_form_fields_filters');function shipping_instance_form_fields_filters(){    $shipping_methods = WC()->shipping->get_shipping_methods();    foreach($shipping_methods as $shipping_method) {        add_filter('woocommerce_shipping_instance_form_fields_' . $shipping_method->id, 'shipping_instance_form_add_extra_fields');    }}function shipping_instance_form_add_extra_fields($settings){    $settings['shipping_extra_field'] = [        'title' => 'Shipping extra field',        'type' => 'text',         'placeholder' => 'shipping',        'description' => ''    ];    return $settings;} 

Thanks @Wprog_dy for idea, but your code only adds field to the 'flat_rate' shipping method and your function is really weirdly complicated.

My example will add a custom field to all shipping methods

this is how I implemented the custom description field in shipping methods with flat rate and free shipping

My function.php file:

add_filter( 'woocommerce_shipping_instance_form_fields_flat_rate', array( $this, 'add_extra_fields_in_flat_rate' ), 10, 1);public function add_extra_fields_in_flat_rate($settings)    {        $counter = 0;        $arr = array();        foreach ($settings as $key => $value) <br>        {            if($key=='cost' && $counter==0)            {                $arr[$key] = $value;                 $arr['shipping_extra_field'] = array(                    'title'         => __( 'Shipping Extra Field', 'woocommerce' ),                     'type'             => 'text',                     'placeholder'    => 'shipping',                    'description'    => ''                );                 $counter++;             }             else             {                $arr[$key] = $value;            }         }        return $arr;     } 

Matic Jan did excelent job and answered this question, but if you can add custom field to any shipping method ... then how do you "use" its content at the checkout page? Here is a code I use, hooking to woocommerce_after_shipping_rate.

function shipping_instance_custom_desc( $shipping_rate, $index ) {    if( is_cart() ) return; // Exit on cart page                $current_instance_ids = WC()->session->get( 'chosen_shipping_methods' );    $current_instance_id = $current_instance_ids[0];            if( $shipping_rate->id == $current_instance_id ) {                $option_key = 'woocommerce_'.$shipping_rate->method_id.'_'.$shipping_rate->instance_id.'_settings';                $instance_settings = get_option( $option_key );                if( isset( $instance_settings[ 'shipping_extra_field' ] ) ) {                        ?>            <div class="shipping-method-desc">                <?php echo $instance_settings[ 'shipping_extra_field' ] ?>            </div>            <?php                    }            }    }add_action( 'woocommerce_after_shipping_rate', 'shipping_instance_custom_desc' , 10, 2 );

I hoped there will be some wc_ function to get shipping method instance settings, but oh boy was I wrong ... I found out shipping method instance settings are stored in wp_options table as any other WP settings. So my code gets this option value and check if it has custom field ( shipping_extra_field, same as Matic Jan used) ... and output it for currently selected shipping method.