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As you have seen, FileMaker Go on iOS has some differences in the behavior of the web viewer object. These are related to the operating systems being very different for implementing a web viewer.
Instead of the method you are using, I would create a container field set to global storage, in order to temporarily display a file. Then, use the Insert File, Insert From URL or another Insert function in order to temporarily import the remote file into the database for display.
I'm not sure that it's possible at all to authenticate as part of a URL in a webviewer, usually plain text passwords are highly discouraged for security reasons. If you are using OAuth, it may be possible to authenticate as part of the file connection to have access to those resources when you need them.