First, if you want, set the border style to None.
The drop the following two functions into the declaration section of the form:
Private Declare Function CreateRoundRectRgn Lib "gdi32" (ByVal X1 As Long, ByVal Y1 As Long, ByVal X2 As Long, _
ByVal Y2 As Long, ByVal X3 As Long, ByVal Y3 As Long) As Long
Private Declare Function SetWindowRgn Lib "user32" (ByVal hwnd As Long, ByVal hRgn As Long, _
ByVal bRedraw As Boolean) As Long
Then in the form load add the following:
SetWindowRgn hwnd, CreateRoundRectRgn(5, 5, 395, 395, 100, 100), True
You can play around with the values in CreateRoundRectRgn to get different results than my example