Dim hwndWin As Long
Dim hwndText As Long
Dim hwnd1 As Long
 
  hwndWin = FindWindow(vbNullString, "Fenstertitel") 
  hwndText = FindWindowEx(hwndWin, 0&, "ThunderRT6TextBox", vbNullString) _
    'Klassenname laut Spy++
  hwnd1 = FindWindowEx(hwndText, 0&, "Text1", vbNullString) 'Controlname laut 
  ' VB Decompiler
 
  Dim t As String
  t = "12"
  SendMessage hwnd1, WM_SetText, 0&, ByVal t

SendMessage hwndtext, WM_SetText, 0&, ByVal t '--> die Klasse ansprechen