How to get Sharepoint credentials to be used in an ASP Net Core app?

I have an ASP NET Core app that will be acessible through a Sharepoint page, once it's published to an IIS Server. I need to get the credentials used to access that page (Windows Authenticaton), because they will then be used to access a third link. I'm using the Razor Pages approach, so I have no controller.

Is there a way to do this?