I needed to open web.config file from the external application(setup utility).
I decided to use WebConfigurationManager.OpenWebConfiguration(path,site) function.
In the documentation it is not clear, that path should started with “\“ , e.g “\MyWebApp“.Passing “MyWebApp“ without leading “\” causes error.
If I want to use metabase site path (e.g "/LM/W3SVC/1") instead of name of the application Web site, I can to pass “1“ as site parameter and it is fully acceptable(but not documented).