ForEach Lamda expression c#

Question

How do you make this into 1 line of code:

foreach (var item in List.Where(x => x.IsSelected == true))
                    {
                        item.box= "done";
                        
                    }

something like this, but that syntax is wrong.

List.ForEach(x=>x.box = done, List.Where(x=>x.IsSelected ==true));