Cannot embed a Power BI Report into a .NET MVC application

[Lorenz33]

We are running Power BI Report Server (May 2019) on SQL Server 2016 Build 13.0.5366.0. The operating system is Windows Server 2016 version 1+07 (OS Build 14393.3085).

We are creating reports using Power BI optimized for Report Server (May 2019).

We have a .NET MVC application and are embedding a Power BI report into a Razor page using the following iFrame tag:

<iframe style="width:100%; height:400px;" src="http://samplereportserver/myreports/powerbi/env/thegreatreport?rs:embed=true" frameborder="0" allowFullScreen="true"></iframe>

When the appliation tries to access the report, it gets asked for login credentials. The account accessing the report does have browser permissions to the report. It appears that the credentials are not being sent across. How can a report be embedded in this environment without having the user prompted for credentials?

I see there is an embed option but it appears only available in the Power BI Service:

https://powerbi.microsoft.com/en-us/blog/easily-embed-secure-power-bi-reports-in-your-internal-porta...

Any advice or help would be greatly appreciated. Thanks.