php calling problem

Question

<?php

/**

 * File: demo.php

 * User: Administrator

 * Date: 2019/5/15

 */

##require_once dirname(__FILE__) . '/lib/Api.php';

/**

* Change to your own account/password

*/

$username = 'Account';

$password = 'Password';

$api = new Api($username, $password);

##/**

* Domain name

*/

$data = array(

'action' => '/oneprice/?act=buy',

'method' => 'POST',

'body' => array(

'domain' => 'Domain name',

'price' => 500,

)

);

## $resp = $api->fetch_api($data);

echo $resp;

'domain' => 'Domain name',

How to replace "domain name" with a callable interface https://www.test.com/?domain=domain name