Permissions problem for dynamically named web data source

I have a PBI report that accesses delimited text files from a web server. Every day a new text file is added to the directory and the file name contains string representing the date. I have created a function to access these files. I have to first construct the name(s) of the file(s) I want. So if I want to get a file generated yesterday, I will calculate the proper date string based on DateTime.LocalNow() and adjust for -1 day. I then use Web.Contents() with my properly constructed file name. This works when I am running the query from Power BI Desktop for Report Server. However when I save the report to the Report Server, and try to set up automatic refresh it fails. The dyanamic data source is not "seen" as a source for which I can enter credentials. Apparently the server expects to have crendentials set for every individual file, but the file names are not constructed until run time. Due to the hand-edited functions and queries, these sources are "invisible" at the time of entering credentials.

I would like to be able to enter credentials for the web folder that the files are in, and have it apply those permissions to any file in the folder. I can't see a way to do this.

Why does the refresh work from PBI Desktop but not from the report running on the report server? And does anyone have a suggestion of how to solve this problem? Thanks for any clues you can provide!