Turn Discount Code dropdown on the PMPro orders list to a select2 field.

my_add_select2_to_discount_code_filter_on_orders_page.php

/**
 * Turn Discount Code dropdown on orders list to a select2 field.
 * Add this code to a custom plugin. https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */
function my_add_select2_to_discount_code_filter_on_orders_page() {
	if ( ! empty( $_REQUEST['page'] ) && $_REQUEST['page'] == 'pmpro-orders' ) {
		wp_enqueue_style( 'select2', plugins_url( 'css/select2.css', __FILE__ ), '', '4.0.6', 'screen' );
		wp_enqueue_script( 'select2', plugins_url( 'js/select2.js', __FILE__ ), array( 'jquery' ), '4.0.6' );
		
		function my_add_select2_inline_script() {
		?>
		<script>
			jQuery(document).ready(function() {
				jQuery('#discount_code').select2();
			});
		</script>
		<?php
		}
		add_action( 'admin_footer', 'my_add_select2_inline_script' );
	}
}
add_action( 'admin_enqueue_scripts', 'my_add_select2_to_discount_code_filter_on_orders_page' );