Instead of using KeyDown, use KeyUp. It shall work. I have tried it out. The following code snippet should do it.
private void Form1_KeyUp(object sender, KeyEventArgs e)
{
if (e.KeyData.ToString().Equals("F6, Control"))
{
Cotation ct = new Cotation();
ct.MdiParent = this;
ct.Show();
}
}
Hope this helps.