Populate Woocommerce order notes with custom field value

Try this:

    add_filter( 'woocommerce_checkout_fields', 'webendev_woocommerce_checkout_fields' );
/**
 * Change Order Notes Placeholder Text - WooCommerce
 * 
 */
function webendev_woocommerce_checkout_fields( $fields ) {

    $fields['order']['order_comments']['placeholder'] = 'Your custom placeholder';
    return $fields;
}