XML : Chargify XML for Custom Field Wordpress

I am using chargify plugin. I want added a custom field in chargify account want to save data using XML.

Presently XML is like which is used to send data to chrgify:-

  <?xml version="1.0" encoding="UTF-8"?>          <subscription>              <product_id>' . $_POST["submit"] . '</product_id>              <customer_attributes>              <first_name>'.$_POST["chargifySignupFirst"].'</first_name>              <last_name>'.$_POST["chargifySignupLast"].'</last_name>              <email>'.$_POST["chargifySignupEmail"].'</email>          </subscription>    

Please let me know how to add custom field in this xml so that it automatically get saved in chargify.

Thanks in advance.