Only show the discount code fields if a code is passed in via URL paramter

hide_my_pmpro_show_discount_code.php

<?php
/**
 * Only show the discount code fields if a code is passed in via URL paramter
 *
 * You can add this recipe to your site by creating a custom plugin
 * or using the Code Snippets plugin available for free in the WordPress repository.
 * Read this companion article for step-by-step directions on either method.
 * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */
function hide_my_pmpro_show_discount_code( $show ) {
	if ( empty( $_REQUEST[ 'discount_code' ] ) ) {
		$show = false;
	}
	return $show;
}
add_filter( 'pmpro_show_discount_code', 'hide_my_pmpro_show_discount_code');

This recipe is included in the blog post on "Hide the Discount Code Fields" at Paid Memberships Pro here: https://www.paidmembershipspro.com/hide-the-discount-code-fields/