Explanation:
To identify which Power BI report file was shared by User1, you should configure the search with the following parameters:
* Activities: Shared Power BI report
* Record Type: PowerBiAudit
* Workload: PowerBi
These parameters will filter the search results to show only the events where a Power BI report was shared by a user in your organization. You can then look for the event that has User1 as the user ID and an external user as the recipient. The event details will show the name and URL of the Power BI report file that was shared.
For more information, see Search the audit log for events in Power BI and Search for content in the Microsoft Purview compliance portal.