Adding multiple config options to a product with API AddOrder

[Bruce]

Hi there - I'm struggling to understand the relationship between "1" and 999 below in 'configoptions' - what specifically does the "1" and the 999 reference? If I wanted to add 2 config options to a single product (say 123 and 456) - what would the below configoptions line change to?
 

http_build_query(
        array(
            'action' => 'AddOrder',
            // See https://developers.whmcs.com/api/authentication
            'username' => 'IDENTIFIER_OR_ADMIN_USERNAME',
            'password' => 'SECRET_OR_HASHED_PASSWORD',
            'clientid' => '1',
            'pid' => array(1,1),
            'domain' => array('domain1.com', 'dómáin2.com'),
            'idnlanguage' => array('', 'fre'),
            'billingcycle' => array('monthly','semiannually'),
            'addons' => array('1,3,9', ''),
            'customfields' => array(base64_encode(serialize(array("1" => "Google"))), base64_encode(serialize(array("1" => "Google")))),
            'configoptions' => array(base64_encode(serialize(array("1" => 999))), base64_encode(serialize(array("1" => 999)))),
            'domaintype' => array('register', 'register'),
            'regperiod' => array(1, 2),
            'dnsmanagement' => array(0 => false, 1 => true),
            'nameserver1' => 'ns1.demo.com',
            'nameserver2' => 'ns2.demo.com',
            'paymentmethod' => 'mailin',
            'responsetype' => 'json',
        )
    )

dsdsds 

[Bruce]

Here are the config options I specifically want to add as quantity: 1:

 "configoptions": {
                    "configoption": [
                        {
                            "id": 866,
                            "option": "Add-on 1 of 2",
                            "type": "quantity",
                            "value": 0
                        },
                        {
                            "id": 865,
                            "option": "Add-on 2 of 2",
                            "type": "quantity",
                            "value": 0
                        },

It seems it works to add 1, with the serialized array as: a:1:{i:855;i:1;} or a:1:{i:856;i:1;} - as soon as I try add both with a:2:{i:855;i:1;i:856;i:1;} it adds neither??