Private Declare Function FindWindow Lib "user32" Alias "FindWindowA" ( _
 ByVal strClassName As String, _
 ByVal lpWindowName As Any) As Long

Private Sub CommandFindWindow_Click()
  Dim hwndTarget As Long
  hwndTarget = FindWindow(vbNullString, "My proga name")
  MsgBox hwndTarget
End Sub

Private Sub CommandFindEnum_Click()
  Dim hwndTarget As Long
  hwndTarget = FindMyWindow
  MsgBox hwndTarget
End Sub

Public Function FindMyWindow() As Long
  hwndEnumTarget = 0
  EnumWindows AddressOf EnumWindowsProc, 1 '1-конкретное окно, пока используем GetWindowText=Form.Caption
  FindMyWindow = hwndEnumTarget
End Function

Private Function EnumWindowsProc(ByVal app_hwnd As Long, _
 ByVal lParam As Long) As Boolean
' Return False to stop the enumeration.
 
  Dim buf As String * 256
  Dim Title As String
  Dim length As Long

  ' Get the window's title.
  length = GetWindowText(app_hwnd, buf, Len(buf))
  Title = Left$(buf, length)
  If lParam = 1 Then 'конкретное окно (пока заданный в Caption текст)
    If Title = "My proga name" Then
      hwndEnumTarget = app_hwnd
      EnumWindowsProc = False ' Save the hwnd and end the enumeration
      '---Debug---
      Debug.Print Title & " (" & app_hwnd & ")"
      Form1.Text1.Text = Title & " (" & app_hwnd & ")"
      '---Debug---
      Exit Function
    End If
  Else 'lparam=0 (все окна)
    '---Debug---
    Debug.Print Title & " (" & app_hwnd & ")"
    Form1.Text1.Text = Form1.Text1.Text & vbCrLf & Title & " (" & app_hwnd & ")"
    '---Debug---
  End If
  
  EnumWindowsProc = True ' Continue the enumeration (default)
End Function

If Title = "My proga name" Then

Button.Caption = "This is the GUID qwertyuiop" 'такую фигню случайно не придумаешь
.Visible=false

    If Title = "Form1" Then
      Dim hwndTemp As Long
      hwndTemp = FindWindowEx(app_hwnd, 0&, vbNullString, "This is the GUID qwertyuiop")
      If hwndTemp <> 0 Then 'если есть child с зарезервированной фигней в его Caption
        hwndEnumTarget = app_hwnd
        EnumWindowsProc = False ' Save the hwnd and end the enumeration
        Exit Function
      End If
    End If

'запрос
Public Sub AskForNewDocumentName()
  hwndTarget = FindWindow(vbNullString, WINDOWTITLE_SERVER)
  If hwndTarget <> 0 Then
    Dim cds As COPYDATASTRUCT
    
    cds.dwData = 2 'идентифицирует тип передаваемых данных -запрос имени документа
    
    Call SendMessage(hwndTarget, WM_COPYDATA, Form1.hwnd, cds)
    
  End If
End Sub

...
'ответ
    Case WM_COPYDATA
      Dim cds As COPYDATASTRUCT
      Dim the_str As String
      
      Call CopyMemory(cds, ByVal lParam, Len(cds))
      
      If (cds.dwData = 2) And (wParam = hwndTarget) Then 'сообщение содержащее имя документа
        the_str = PtrToString_len(cds.lpData, cds.cbData)
        Form1.TextDocumentName.Text = the_str
      End If
      Exit Function