how to dynamically set the data source in power bi service while the user login??

We need to create a COMMON REPORTING SERVICE(PowerBI) with standard reports that work for the business segment
e.g. Fashion for all customers: Max, Trends, Soch.
The need is the run these reports as a reporting service using PowerBI.
The PowerBI reporting server will access the transaction database for each customer in run-time and then deliver a report to the end-users.
We don't have access to the Transactional database. Only during the runtime, based on user login or so, the data source should be dynamically passed
and respective datasets should be connected.
While the users from different customer organizations access the same reports, the data to be shown will be relevant to only the relevant organization’s data
i.e. Max executives see only Max data and cannot see Trends data

Please note, we don't have access to the Client's Datasource of each product.
Also, there is a limitation not to create the Central repository of all the Product's data.
We need to connect to the DB dynamically during runtime in PowerBI Service.
Any suggestions.