Using Windows environment variable in a PowerBI desktop parameter

I have yet to get this to work right.  I don't know why we can't put in %USERNAME% to dynamically shift document locations.  I have hundreds of students that access a PBIX file that is created from an Excel file each sememster.  Getting them to know where to put the excel file is very painful.

What I do, is I create two parametmers.  FilePath and Filename.  I put them in a group called File Location. (1)

Curernt Value for FilePath is C:\ITM310  Filename is name of the excel file used to build the PBIX file.  Put a \ before the file name.  (In my instance the file name doesn't change, but the folder name does.)

Then, for the source (2) in each table, I go in and set the excel workbook to Advanced (3) and use the Filepath Parts.  I pull the parameters for FilePath and Filename.  It then puts them together: {FilePath}{Filename} .(4)

It's not the best method, but it is bette than nothing.  This is something I have been trying to rectify for a few years now.