Add a new action to the order action drop down

woocommerce-custom-order-action.php

add_action('woocommerce_order_actions', 'custom_wc_order_action', 10, 1 );
function custom_wc_order_action( $actions ) {

	if ( is_array( $actions ) ) {
		$actions['custom_action'] = __( 'My custom action' );
	}

	return $actions;

}

/**
 * Filter name is woocommerce_order_action_{$action_slug}
 */
add_action( 'woocommerce_order_action_custom_action', 'fired_my_custom_action' );
function fired_my_custom_action( $order ) {

	die( 'do my action ere' );

}