[DllImport("user32.dll")]
    static extern bool PostMessage(IntPtr hWnd, UInt32 Msg, int wParam, int lParam);

    public const uint WM_SETTEXT = 0x000C;
    const int VK_F5 = 0x74;

    private static void InteropSetText(IntPtr iptrHWndDialog, string strTextToSet)
    {
        ///Step 1
        ///Find all parent controls on SaveAs dialog
        var list = GetChildWindows(iptrHWndDialog);

        ///Step 2
        /// Find control with className = "ComboBox"
        foreach (var item in list)
        {
            string className = GetWindowClass(item);
            
            if (className != "ComboBox")
                continue;

            HandleRef hrefHWndTarget = new HandleRef(null, item);

            /// Step 3
            ///Set text in Control. 
            SendMessage(hrefHWndTarget, WM_SETTEXT, IntPtr.Zero, strTextToSet);

            /// Step 4
            //HACK: Without press F5 button New filename was not applied in Save dialog
            PostMessage(item, WM_KEYDOWN, VK_F5, 0);
            return;
        }
    }