If you upload a credential json to Dropbox or Google Drive and get a public link for it then attempt to verify using that link in the "paste URL or JSON" field it will fail with an error claiming it can't parse the JSON.

What's actually happening appears to be that those links don't have CORS settings that allow embedding. The only way I can thing to make this work would be to have some server side code that will download the JSON to a temporary location and have the page load it from there.