Private Declare Function FindWindow Lib "user32" Alias "FindWindowA" (ByVal lpClassName As String, ByVal lpWindowName As String) As Long
Private Declare Function GetDC Lib "user32.dll" (ByVal hWnd As Long) As Long
Private Declare Function SetPixel Lib "gdi32" (ByVal hdc As Long, ByVal X As Long, ByVal Y As Long, ByVal crColor As Long) As Long

Private Sub Risovalka()
Dim WndTitle As String, WinWnd As Long
Let WndTitle = "Безымянный — Блокнот"
Let WinWnd = FindWindow(vbNullString, WndTitle)
If (WinWnd = 0) Then MsgBox "Нет окна [" & WndTitle & "]": Stop

Dim hdc As Long
Let hdc = GetDC(WinWnd)
    
Do
    DoEvents
    Call SetPixel(hdc, Rnd * 1000, Rnd * 1000, Rnd * 1000)
Loop
End Sub