Private Declare Function SendMessage Lib "user32" Alias "SendMessageA" (ByVal _
   hwnd As IntPtr, ByVal wMsg As Integer, _
           ByVal wParam As Integer, ByVal lParam As Integer) As Integer
 
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As _
      System.EventArgs) Handles Button1.Click
        Const WM_KEYDOWN = &H100
        Dim Handle As IntPtr = TextBox1.Handle
        SendMessage(Handle, WM_KEYDOWN, Keys.Enter, 0)
    End Sub