WooCommerce - Add custom field (in an order) to the order emails

gistfile1.php

/**
 * Add the field to order emails
 **/
add_filter('woocommerce_email_order_meta_keys', 'my_woocommerce_email_order_meta_keys');

function my_woocommerce_email_order_meta_keys( $keys ) {
	$keys['How did you hear about us?'] = 'hear_about_us';
	return $keys;
}

@dimbert82
insert the code in your active theme or child theme functions.php

code like this:

add_filter( 'woocommerce_email_order_meta_fields', 'custom_order_mail_fields_callback', 10, 3 );
function custom_order_mail_fields_callback( $fields, $sent_to_admin, $order ) {    
        $fields['yourdata'] = array(
            'label' => __( 'yourdata to display' ),
            'value' => ' yourdata value',
        );
    }
    return $fields;
}