How to authenticate external users when embedded Report Server through an iFrame on my application?

We need to integrate Power Bi Report Server with our application and replace the current SSRS reports with Power Bi Report server which was integrated with our application through ReportViewer control.
After some research we figured out that the only way to integrate Power Bi Report Server is by using IFrame and that is OK, but how could we authenticate the external users of our application in the Power Bi Server?