Add data to the Pushover New Order notification

functions.php

<?php

/**
 * Hook into Pushover send notification on new order and add something to the title from order.
 *
 * @hook 'wc_pushover_notify_new_order'
 * @param $args
 * @return mixed
 */
function sp_wc_pushover_notify_new_order( $args ) {
	if ( isset( $args['order_id'] ) ) {
		$order_id = $args['order_id'];
		$meta     = get_post_meta( $order_id, '_billing_country', true );
		if ( $meta ) {
      // Add to title
			$args['title'] .= ' Country is ' . $meta;
      // Modify the message
      //$args['message'] = '';
      // Modify the URL
      //$args['url'] = '';
		}    
	}
	return $args;
}
add_filter( 'wc_pushover_notify_new_order', 'sp_wc_pushover_notify_new_order', 10 );

Hooks available:
New Order notification (free) - wc_pushover_notify_free_order
New Order notification - wc_pushover_notify_new_order
Back order - wc_pushover_notify_backorder
No Stock - wc_pushover_notify_no_stock
Low Stock - wc_pushover_notify_low_stock