SendMessage to execute button and wait until it's finished - delphi-xe2

Question

I am using application A to control application B. I used SendMessage to execute a button in application B. After button in application B is executed, application A do another task without waiting application B finish its task. How can I wait a process of click button in application B finished before starting another task of application A?
begin
ParentWindow := FindWindow('TfrmMain', nil);
if (ParentWindow = 0) then
begin
ShowMessage('App cannot be found!!');
Exit;
end;
// Get handle of Combobox
Memo_temP.Clear;
EnumChildWindows(ParentWindow, #EnumChildrenTComboBox, UINT_PTR(Memo_temP.Lines));
temStr := Memo_temP.Lines.Strings[0];
ComboboxHWND := StrToInt64('$' + temStr);
// Get handle of output vaule
Memo_temP.Clear;
EnumChildWindows(ParentWindow, #EnumChildrenTMemo, UINT_PTR(Memo_temP.Lines));
temStr := Memo_temP.Lines.Strings[0];
MemoTxt2HWND := StrToInt64('$' + temStr);
// Get handle of btn
Memo_temP.Clear;
EnumChildWindows(ParentWindow, #EnumChildrenTButton, UINT_PTR(Memo_temP.Lines));
temStr := Memo_temP.Lines.Strings[1];
if (temStr = '') then begin ShowMessage('Error4'); Exit; end;
AsyncRunTestHWND := StrToInt64('$' + temStr);
i := -1;
while True do
begin
Inc(i);
// Select each item in ComboBox
SendMessage(ComboboxHWND, CB_SETCURSEL, i, 0);
SendMessage(ComboboxHWND, WM_COMMAND, MakeWParam(0, CBN_SELCHANGE), ComboboxHWND);
// Click btn
SendMessage(AsyncRunTestHWND, WM_LBUTTONDOWN, MK_LBUTTON, MakeLParam(5, 5));
SendMessage(AsyncRunTestHWND, WM_LBUTTONUP, MK_LBUTTON, MakeLParam(5, 5));
// get result text from memo
BufLen := SendMessage(MemoTxt2HWND, WM_GETTEXTLENGTH, 0, 0);
SetLength(Buffer, BufLen + 1);
SendMessage(MemoTxt2HWND, WM_GETTEXT, BufLen, LParam(PChar(Buffer)));
if (PosEx('Error', Buffer) > 0) then
Memo_Error.Lines.Add(Buffer)
else
Memo_Succeed.Lines.Add(Buffer);
if SendMessage(ComboboxHWND, CB_SETCURSEL, i, 0) < 0 then Break;
end;
end;