post https://api.clientpay.com/Invoice/Forms/Add
In order to create an Invoice Payment Page, you need an invoice object. You can create single or multi-invoice payment pages with the Invoice call. The invoices array, with the Invoice call, can be used to create multiple invoice objects.
If the call is successful, an Invoice Payment Page URL will be returned in order to process a payment. A RequestID is created for the API Invoice Payment Page as a unique identifier.