How can I insert a row into a subform using API V2 and PHP SDK

Looking at the V2 documentation on subforms, I do not see any documentations about how to insert rows into a subform using the PHP SDK.

ie: https://www.zoho.com/crm/developer/docs/api/insert-subforms.html

How would I go about doing that?

I have tried something like this with no success, where "Debt_Matrix" is my subform:

$zcrmRecordIns = ZCRMRecord::getInstance("Contacts", $contactId);
$zcrmRecordIns->setFieldValue("Debt_Matrix", [ 
    [
        'Priority' => 1,
        'Creditor' => 'Creditor',
        'Balance' => 10.00,
        'APR' => 7.99,
        'Payment' => 11.00, 
        'Limit' => 12.00,
        'Notes' => 'NOTES'
    ]
]);
$apiResponse = $zcrmRecordIns->update();