add-discount-code-mailchimp-field.php

<?php
function my_pmpro_mailchimp_listsubscribe_fields($fields, $user){
	
	if( isset( $_REQUEST['discount_code'] ) && $_REQUEST['discount_code'] !== "" ){
		
		$new_fields =  array(
      "DISCOUNTC" => $_REQUEST['discount_code'],	     
    );
	
		$fields = array_merge($fields, $new_fields);

	}

	return $fields;

}

add_action('pmpro_mailchimp_listsubscribe_fields', 'my_pmpro_mailchimp_listsubscribe_fields', 10, 2);