Option Explicit
Private Const WM_SETTEXT = &HC
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 Sub cmdSetText_Click()
Dim sStr$
Dim lpStr&
sStr = "HELLO"
SendMessage txtSetter.hwnd, WM_SETTEXT, 0&, ByVal sStr
End Sub