Форма:

Private Sub Form_Load()

    Dim FormWidthInPixels As Long
    Dim FormHeightInPixels As Long
    Dim a, ret
    
    FormWidthInPixels = Me.Width / Screen.TwipsPerPixelX
    FormHeightInPixels = Me.Height / Screen.TwipsPerPixelY
    
    a = CreateRoundRectRgn( 0 ,  0 , FormWidthInPixels, FormHeightInPixels,  24 ,  24 )
    
    a = SetWindowRgn(Me.hwnd, a, True)

    ret = DeleteObject(a)

End Sub


Модуль:

Public Declare Function SetWindowRgn Lib "user32.dll" (ByVal hwnd As Long, ByVal hRgn As Long, ByVal bRedraw As Boolean) As Long
Public Declare Function CreateRoundRectRgn Lib "gdi32.dll" (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
Public Declare Function DeleteObject Lib "gdi32.dll" (ByVal hObject As Long) As Long