updating from background Paypal subform

updating from background Paypal subform

There's limited access to the paypal subform, it doesn't appear I can pull any of those fields into the reports, so I'm trying to update a form field, this seems simple/easy but isn't working, any thoughts?

On Edit - On Success

if (ZC_Payment.payment_status == "Completed")
{
    input.Payment = "paid";
}