void Form1_Paint(object sender, PaintEventArgs e)
{
PopupContainerBarControl barControl = (sender as PopupContainerBarControl);
Point offsetLocation = new Point(barControl.CloseButton.Location.X + barControl.CloseButton.Width + 3, barControl.CloseButton.Location.Y);
e.Graphics.DrawString("Custom string", barControl.Font, Brushes.Green, offsetLocation);
}
void popupControlContainer1_Popup(object sender, EventArgs e)
{
PopupControlContainer control = sender as PopupControlContainer;
PopupContainerBarControl barControl = (control.Parent as PopupContainerBarControl);
barControl.Paint += Form1_Paint;
}