In every data source connection string, you can add a simple expression that maps the current Windows username to the CUSTOMDATA property of the data source provider. This works in SSRS embedded data sources, shared data sources, in a SharePoint Office Data Connecter (ODC) file and in a SharePoint BISM connection file. In each case, the syntax should be the similar. Here is my shared data source on the SSRS 2016 report server
This is pretty snazzy. Paul goes into good detail on the topic, so read the whole thing.