Creates a new merge field in Mailchimp if one doesn't already exist.

pmpro_mailchimp_merge_fields.php

<?php
// Add Zip to Mailchimp
// Copy from below here...

/*
 * Creates a new merge field in Mailchimp if one doesn't already exist.
 */
function my_pmpro_mailchimp_merge_fields( $merge_fields ) {
	// Adds a "COMPANY" merge field to Mailchimp.
	//$merge_fields[] = array('name' => 'COMPANY', 'type' => 'text');
	'MERGE10'   => ( isset( $_REQUEST['bzipcode'] ) ) ? sanitize_text_field( $_REQUEST['bzipcode'] ) : get_user_meta( $user->ID, 'bzipcode', true ),
	return $merge_fields;
}
add_filter( 'pmpro_mailchimp_merge_fields', 'my_pmpro_mailchimp_merge_fields' );