諸般の事情により、久し振りにVBAを使ったなー。

Module1.bas

Option Explicit

Declare PtrSafe Function FindWindow Lib "user32" Alias "FindWindowA" (ByVal lpClassName As String, ByVal lpWindowName As String) As LongPtr
Declare PtrSafe Function SendMessage Lib "user32" Alias "SendMessageA" (ByVal hWnd As LongPtr, ByVal wMsg As Long, ByVal wParam As LongPtr, lParam As Any) As LongPtr


Sub hoge()

    Dim hWnd&: hWnd = FindWindow("Notepad", vbNullString)
    SendMessage hWnd, &H111, 4, ByVal 0&


End Sub