How to change MsgBox message caption at runtime?

Question

I need to change the default caption of a MsgBox message box at runtime. Currently it constantly shows the value of the SetupAppTitle directive as

Answer 1

This is how i finally did it:

[Code]
{ https://msdn.microsoft.com/en-us/library/windows/desktop/ms645505.aspx }
{ Use Windows MessageBox() function as an MsgBox() replacement. }
{ MessageBoxW is the UNICODE version of this API call. }
const
  { these are not exported in Inno Setup! }
  MB_ICONERROR = $00000010;
  MB_ICONWARNING = $00000030;
  MB_ICONINFORMATION = $00000040;
  MB_ICONQUESTION = $00000020;

function _MessageBoxW_(hWnd: Integer; lpText, lpCaption: String; uType: Cardinal): Integer;
  external 'MessageBoxW@user32.dll stdcall';

{ Usage: SysMsgBox('Error', 'Shit happens!', MB_OK or MB_ICONERROR); }
{        res =: SysMsgBox('Question', 'blah blah', MB_YESNO or  MB_ICONQUESTION); }
function SysMsgBox(const Caption, Message: String; const Flags: Integer): Integer;
begin
  Result :=
    _MessageBoxW_(StrToInt(ExpandConstant('{wizardhwnd}')), Message, Caption, Flags);
end;

Thanks to all for your help!