Example of using the pmpro_mailchimp_listsubscribe_fields filter to send extra fields to MailChimp.

sidetrack_mailchimp_listsubscribe_fields.php

/*
	Sync fields to MailChimp
*/
function sidetrack_mailchimp_listsubscribe_fields($fields, $user)
{
	$new_fields =  array(
      "NEWSLETTER SELECTION" => $user->sidetrack_newsletter_selection);

	$fields = array_merge($fields, $new_fields);
	  
	return $fields;
}
add_action('sidetrack_mailchimp_listsubscribe_fields', 'sidetrack_mailchimp_listsubscribe_fields', 10, 2);

/*
	(Optional) Tell PMPro MailChimp to always synchronize user profile updates. By default it only synchronizes if the user's email has changed.
	
	Requires PMPro Mailchimp v2.0.3 or higher.
*/
add_filter('pmpromc_profile_update', '__return_true');