Example on how to add custom fields to constant contact using Inbound Now's Constant Contact extension and hooks.

functions.php

<?php
/**
* This code snippit will attempt to add 2 custom fields to the ConstantContact contact data array before being sent to the API
* @author Hudson Atwell
*/

add_filter( 'inboundnow_constantcontact_lead_data' , 'add_custom_field' , 10 , 1 );
function add_custom_field( $contact ) {
 
  /* You may have to look at all the key/value pairs in the $_POST variable to discover the correct key name */ 
  
  /* Due to limitations in ConstantContact's API we have to name our custom fields in this format: CustomFieldN where N is 1-15 */
  $customfield_1 = array( 'name' => 'CustomField1' , 'value' => $_POST['custom-field-input-name-here-1'] ) ;
	$customfield_2 = array( 'name' => 'CustomField2' , 'value' => $_POST['custom-field-input-name-here-2']) ;
	
	/* ConstantContact support recommended changing the arrays to objects, I did not try differently */
	$contact['custom_fields'] = array( 
		(object) $customfield_1 ,
		(object) $customfield_2
	);
	
	/* Return the updated contact array to our CC extension before sending it to the CC API */
	return $contact;
}