I answered a similar question many years ago on vbforums.com, but could not find it, so I quickly rewrote it for you. I commented on the code so that you do not have problems understanding it.
Like you and I, we have names, similarly, windows have descriptors (hWnd), a class, etc. Once you know what hWnd is, it’s easier to interact with it. The Findwindow API finds the hWnd of a particular window using the class name. Read the rest of the API here.
Private Declare Function FindWindow Lib "user32" Alias "FindWindowA" _
(ByVal lpClassName As String, ByVal lpWindowName As String) As Long
Private Declare Function FindWindowEx Lib "user32" Alias "FindWindowExA" _
(ByVal hWnd1 As Long, ByVal hWnd2 As Long, ByVal lpsz1 As String, _
ByVal lpsz2 As String) As Long
Private Declare Function SendMessage Lib "user32" Alias "SendMessageA" _
(ByVal hwnd As Long, ByVal wMsg As Long, ByVal wParam As Long, lParam As Any) As Long
Private Const WM_SETTEXT = &HC
Private Sub Command1_Click()
Dim Ret As Long, ChildRet As Long
Dim sString As String
'~~> This is the value from the cell which
'~~> you want to send to notepad
sString = Range("A1").Value
'~~> Start Notepad
Ret = Shell("notepad.exe", vbNormalFocus)
'~~> Wait for it to load
DoEvents
'~~> Find notepad
Ret = FindWindow(vbNullString, "Untitled - Notepad")
'~~> Check if found
If Ret = 0 Then
MsgBox "Cannot find Notepad Window"
Exit Sub
End If
'~~> Find the "Edit Window" which is a child window of Notepad window
ChildRet = FindWindowEx(Ret, ByVal 0&, "Edit", vbNullString)
'~~> Send the message
SendMessage ChildRet, WM_SETTEXT, 0, ByVal sString
End Sub
, Spy ++ uuSpy. , "" Spy ++
