Declare Function SendMessage Lib "user32" Alias "SendMessageA" _
    (ByVal hwnd As LongLong, ByVal wMsg As Long, _
    ByVal wParam As LongLong, lParam As Any) As LongLong

Option Explicit

Type COPYDATASTRUCT
    dwData As Long
    cbData As Long
    lpData As Long
End Type

Declare Function SendMessage Lib "user32" Alias "SendMessageA" _
    (ByVal hwnd As Long, ByVal wMsg As Long, _
    ByVal wParam As Long, lParam As Any) As Long

Const WM_COPYDATA = &H4A

Dim hwnd As Long
Dim cds As COPYDATASTRUCT
Dim sMsg As String
    
hwnd = FindWindow("XLMAIN", "Microsoft Excel - TR3.xlsm")

          sMsg = "письмо"

          cds.dwData = 0
          cds.cbData = Len(sMsg) * 2 + 2
          cds.lpData = StrPtr(sMsg)
          i = SendMessage(hwnd, WM_COPYDATA, 0&, cds)

Public Sub Hook() 
 lpPrevWndProc = SetWindowLong(gHW, GWL_WNDPROC, _
          AddressOf WindowProc)
          Debug.Print lpPrevWndProc
      End Sub