I want to save a int like this:
Properties.Settings.Default.Score = ScoreInt;
So when I reopen the app (it's a clicker game) the int should be the one the player has made.
Add an event handler for closing your app and add the call
Properties.Settings.Default.Save();
And in the program startup do the assignment like Robert placed in the comments
ScoreInt = Properties.Settings.Default.Score;