What is the syntax for the 'Query string value' in a view argument input?
Syntax depends on your view display path, e.g. for page displays.

For example, if your page display path is url_to_view_page/% then the syntax is: 

url_to_view_page/foo.

If you do not have a path, then you'll always fall back on the contextual filter not provided behavior, this is always the case for blocks.