Hello Reza, Thank you for your valuable sessions.
I plan to setup write back, however my data source is a Salesforce report and not an excel file stored on my hard disk!
I also have gone so far to connect a table in my dashboard with the powerapp and it reacts to any change of the table. So far so good. 
The problem: I have added 3 empty column in the data model that I would like to use and write back. By creating a form in power Apps, I cannot find any place to connect my power Bi model and choose the fields that I would like to use for write back. It would work, if the data source were i.e. an XLS file. Any idea? Many thanks, Reza