See the question and my original answer on StackOverflow

There is no documented way, you'll have to use your own editor. But you can derive from the standard editor class. Here is an example of such a hack:

Define the custom editor attribute like this on the collection property:

[Editor(typeof(MyCollectionEditor), typeof(UITypeEditor))]
public List<Child> Children { get; }

With this editor code:

// CollectionEditor needs a reference to System.Design.dll
public class MyCollectionEditor : CollectionEditor
    public MyCollectionEditor(Type type)
        : base(type)

    protected override CollectionForm CreateCollectionForm()
        CollectionForm form = base.CreateCollectionForm();
        var addButton = (ButtonBase)form.Controls.Find("addButton", true).First();
        addButton.Click += (sender, e) =>
                MessageBox.Show("hello world");
        return form;

The add button is a simple Winforms button, so you can do anything with it.