How do i paint a rectangle onto a panel when a button is clicked?

[KenBZim]

I know how to paint a rectangle i.e.

Private Sub Panel1_Paint(ByVal sender As System.Object, ByVal e As System.Windows.Forms.PaintEventArgs) Handles Panel1.Paint
Dim R As New Rectangle(120, 142, 440, 56)
e.Graphics.DrawRectangle(Pens.BlueViolet, R)
End Sub

But how do i get this to take place only when the user clicks a button?