I think concept of DWA (Directed Web Access) will be apt for your requirement.
Directed Web Access (DWA) is a capability that supports external assignments in a case. By collecting information from stakeholders who are outside your organization, you can extend the scope of your application. It allows anyone accessing the internet, an intranet, or email to process an assignment, either on a one-time or infrequent, as-needed basis.
For more information on DWA please review the below pega help URLs.
Thanks for reply. Passing username/password is not possible as onboarding that user is lengthy process. So that's the reason we are using IAC. I could see some options in Gateway url. But not sure how to do this dynamically for each and every case or assingment.