How to save a List<string> on Settings.Default?

No problem at all! Create a new setting, e.g. "MyListOfStrings", type doesn't matter.

then open settings file in a xml editor

your file will look like this:

now change it as shown below and save it

well, that's all, now it will look like that:

and in code:

I found out that I can't directly save a List<string> on the application settings, but I saw that I can save a StringCollection.

And here I found out that it's very simple to convert from a StringCollection to a List<string>

var list = stringCollection.Cast<string>().ToList();