Accessing Public Shared Point Folder

You need to use authentication with SharePoint sources for Power BI. The "public" access sharing is via sharing link, which won't work in the SharePoint connectors. What I would do is have the organization "share" access to the content with your Microsoft account explicitly via granting actual permission on the files or site (preferably the site). If you can get added as a site member, that would be more foolproof. I just tested this with my external account on my tenant and it worked (I wasn't sure if it'd work for external users - looks like it does!). 

Then you need the site URL or actual file path to connect (depending on the connector you're using), which will be a little bit of a different format from the one you have now. It won't have the :f/s in it, it usually has a format like https://rootname.sharepoint.com/sites/sitename instead.