At the moment I can't save a pbix on the server, if the server is configured with a custom authentication (form). Even if it would be possible the live connection to ssas would not work because there...
Thanks for your feedback. The June 2017 release of Power BI Report Server does work with custom authentication extensions (the May 2017 Preview didn't). Although Power BI Desktop doesn't support this authentication method, you can save your *.pbix file to local disk and then upload it to the Report Server web portal. (On this topic, while we support SQL Server Reporting Services-based custom authentication/authorization extensions today, we hope to invest in OpenID Connect/OAuth in future as a more modern approach to extensible authentication/authorization.)
