Wordpress-在提交给post.php时获得$ _REQUEST的值

Question

I am using WooCommerce on wordpress and have subscribed to a filter to add some menu options to the "Order Actions" list in the top right hand corner of an order.

This form submits to post.php with a select name of wc_order_action. How can I grab the selected value? I've done a print_r($_REQUEST) and apart from the 2 query string parameters, there is nothing else listed.