Yes, it is possible to pass a URL of a GraphML file to yEd Live to open the file automatically.
The format for directly loading graphs via URL is https://www.yworks.com/yed-live/#file=URL where URL is, well, the URL of the GraphML file you want to open.
Be aware that the file must be cross origin accessible (i.e. Access-Control-Allow-Origin: * must be set on the response's file header).
For instance, this works for files shared by a public link via Dropbox while shared links of Google Drive and OneDrive are more restrictive.