<?php
$customer = array(
            'customer' => array(
                'first_name' => 'first',
                'last_name' => 'last',
                'email' => 'email@email.com',
                'verified_email' => true,
                'metafields' => array(
                    0 => array(
                        'key' => 'affiliateID',
                        'value' => 'refid',
                        'value_type' => 'single_line_text_field',
                        'namespace' => 'global'
                    )
                )
            )
        );
echo json_encode($customer);