I can't find anything on Google about this, I am using the Plesk XML API with PHP but when I use this code:
//Add to Plesk
require("../../plesk-php-api.php");
$client = new PleskApiClient('broomfieldhosting.co.uk');
$client->setCredentials('admin', 'adminpassword');
//Webspace
$pleskDomainRequest = <<<EOF
<packet>
<webspace>
<add>
<gen_setup>
<name>flamingfireball64.com</name>
<htype>vrt_hst</htype>
<ip_address>104.238.170.144</ip_address>
</gen_setup>
<hosting>
<vrt_hst>
<ip_address>104.238.170.144</ip_address>
</vrt_hst>
</hosting>
<plan-name>pro</plan-name>
</add>
</webspace>
</packet>
EOF;
$pleskDomainResponseXML = $client->request($pleskDomainRequest);
I get the following error:
Error 2204: Unable to update hosting preferences. System user update is failed: Unable to check system user existance: login name is empty. Incorrect fields: "login".
As I understand you would like to create a subscription flamingfireball64.com. You forgot to specify system user name and its password. Below API can be used to create a subscription. Replace my data with yours.