how to textbox anywhere on chart in c#

RA = new RectangleAnnotation();
RA.BackColor = Color.LightPink;
RA.Alignment = ContentAlignment.TopLeft;
chart.Annotations.Add(RA);
RaPos();

void RaPos()
{
    if (RA == null) return;
    ElementPosition LP = chart.Legends[0].Position;
    RA.X = LP.X;
    RA.Y = LP.Bottom + 5;  // 5% below the legend
    RA.Width = LP.Width;
    RA.Height = 100 - LP.Bottom  - 10;  // leave 10% of the remaining space
}