Customizing the windows authentication in Power BI Page

Hello,
Did any one change the login screen for Power BI reports. Currently whenever I logined to power BI report page , it open the window login prompt.

I try to see this in IIS server for default website but it appears no site to me over there.

I need to change the login page for the same from default windows login to a custom login page.

Currently microsoft provided that they are using the windows based Authentication to Report server. I need to cuaotmize this to new login screen but authentication should be windows based which is already there.