property focusedBox

This property get and sets the box or control that has keyboard tab focus. If no control has keyboard input focus, then null is returned. focusedBox lets you set tab focus to a control (that accepts it)..or query whatever box if any has tab focus.

function onSample()
{
    var focusedBox = scene.focusedBox;
    if (focusedBox)
    {
        shell.print(focusedBox.id);
    }
}