I created a web experience and am currently trying to lock it behind a login for some of the city clients. However I am hosting it through our IIS so I can have a custom web URL for the experience. I am getting the following login as the popup:

We have our Portal through a SAML login for most of our users. I was wondering what is prompting this login popup instead of the standard ArcGIS login/SAML login screen that we normally see.