ConfigurationManager.AppSettings generates compile time error on .Net Standard 1.5

Question

I'm using .Net Standard 1.5 and want to read some values out of my config. However the following line

ConfigurationManager.AppSettings["Foo"]

gives me the following compile time error:

CS7069 Reference to type 'NameValueCollection' claims it is defined in 'System', but it could not be found

I thought that maybe in .Net Standard there is another way to read from AppSettings, but I haven't found anything on that topic.