I am certainly not a pfSense expert, therefore I cannot tell you if pfSense can actually work as a standalone external captive portal/hotspot solution.
Probably best to check out the official documentation (https://www.netgate.com/docs/pfsense/book/captiveportal/index.html) or talk post the question to the pfSense community.
In the context of using IAP with an external captive portal, whatever the captive portal is (pfSense in your case), you need
- a captive portal
- an authentication server (i.e. to enable MAC caching use cases)
This video explains how you configure Instant with the Aruba ClearPass solution as external captive portal: https://www.youtube.com/watch?v=5sQIKZw5BrE&t=336s
While Aruba of course closely integrates their own solutions with eachother, the elements you see there would need to be provided by your pfSense or an alternative. Maybe with what you know about pfSense, you can customize the workflow to work in a similar fashion.