AddBillableItem

$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, 'https://www.example.com/includes/api.php');
curl_setopt($ch, CURLOPT_POST, 1);
curl_setopt($ch, CURLOPT_POSTFIELDS,
    http_build_query(
        array(
            'action' => 'AddBillableItem',
            // See https://developers.whmcs.com/api/authentication
            'username' => 'IDENTIFIER_OR_ADMIN_USERNAME',
            'password' => 'SECRET_OR_HASHED_PASSWORD',
            'clientid' => '1',
            'description' => 'This is a billable item',
            'amount' => '10.00',
            'invoiceaction' => 'recur',
            'recur' => '1',
            'recurcycle' => 'Months',
            'recurfor' => '12',
            'duedate' => '2021-01-01',
            'responsetype' => 'json',
        )
    )
);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
$response = curl_exec($ch);
curl_close($ch);

$command = 'AddBillableItem';
$postData = array(
    'clientid' => '1',
    'description' => 'This is a billable item',
    'amount' => '10.00',
    'invoiceaction' => 'recur',
    'recur' => '1',
    'recurcycle' => 'Months',
    'recurfor' => '12',
    'duedate' => '2021-01-01',
);
$adminUsername = 'ADMIN_USERNAME'; // Optional for WHMCS 7.2 and later

$results = localAPI($command, $postData, $adminUsername);
print_r($results);

{
    "status": "success",
    "billableid": 1
}