Initializes a hosted form payment session with all the necessary data (order information, customer information, payment method...).
This API route can be used for classic hosted form. In that case, this operation is an alternative to the "client redirection POST" method (with HMAC).
It's also the standard way for initializing a payment session for "iframe integration".
You will get an URL pointing to a paymentform, so that the customer is able to complete the payment form with his card details.
You will also get a PaymentSessionId, useful for calling the GetPaymentResult API when the payment is completed.