To get the URL (address for payment), you need to send a request to the IPS Assist server using the HTTP POST method (in UTF-8 encoding). You can redirect the payer to the URL received in the response to complete the payment. Also, the URL received in the response can be substituted into the iFrame address.
At the moment of addressing the PayRequest service, an order is created on the IPS Assist side and the payment process actually begins (from this moment the payment timeout is counted). Therefore, the time interval between the call to the PayRequest service and the direct redirection of the payer should be minimal.