Add custom checkbox to WooCommerce checkout

gistfile1.txt

//Source: https://stackoverflow.com/a/45911284/898933

// Add custom checkout field: woocommerce_review_order_before_submit
	add_action( 'woocommerce_review_order_before_submit', 'my_custom_checkout_field' );
	function my_custom_checkout_field() {
		echo '<div id="my_custom_checkout_field">';

		woocommerce_form_field( 'my_field_name', array(
			'type'      => 'checkbox',
			'class'     => array('input-checkbox'),
			'label'     => __('My custom checkbox'),
		),  WC()->checkout->get_value( 'my_field_name' ) );
		echo '</div>';
	}

	// Save the custom checkout field in the order meta, when checkbox has been checked
	add_action( 'woocommerce_checkout_update_order_meta', 'custom_checkout_field_update_order_meta', 10, 1 );
	function custom_checkout_field_update_order_meta( $order_id ) {

		if ( ! empty( $_POST['my_field_name'] ) )
			update_post_meta( $order_id, 'my_field_name', $_POST['my_field_name'] );
	}

	// Display the custom field result on the order edit page (backend) when checkbox has been checked
	add_action( 'woocommerce_admin_order_data_after_billing_address', 'display_custom_field_on_order_edit_pages', 10, 1 );
	function display_custom_field_on_order_edit_pages( $order ){
		$my_field_name = get_post_meta( $order->get_id(), 'my_field_name', true );
		if( $my_field_name == 1 )
			echo '<p><strong>My custom field: </strong> <span style="color:red;">Is enabled</span></p>';
	}