Schedule Refresh from Excel stored on External SharePoint Site (not SP online)

I am able to get data into Power BI Desktop from an Excel spreadsheet stored on SharePoint but I have not been able to schedule refreshes after I publish a report from it because I cannot edit my credentials successfully.  The SharePoint site is external to me (not SP Online) and I use a user name and password to connect from Power BI Desktop.

Any ideas are appreciated.  Thanks.