const
   InputBoxMessage = WM_USER + 200;

type
   TForm1 = class(TForm)
     Button1: TButton;
     procedure Button1Click(Sender: TObject);
   private
     procedure InputBoxSetPasswordChar(var Msg: TMessage); message InputBoxMessage;
   public
   end;

var
   Form1: TForm1;

implementation

{$R *.DFM}

procedure TForm1.InputBoxSetPasswordChar(var Msg: TMessage);
var
   hInputForm,hEdit,hButton: HWND;
begin
   hInputForm := Screen.Forms[0].Handle;
   if (hInputForm <> 0) then
   begin
     hEdit := FindWindowEx(hInputForm,'TEdit',nil);
     {
       // Change button text:
       hButton := FindWindowEx(hInputForm,'TButton',nil);
       SendMessage(hButton,WM_SETTEXT,Integer(PChar('Cancel')));
     }
     SendMessage(hEdit,EM_SETPASSWORDCHAR,Ord('*'),0);
   end;
end;

procedure TForm1.Button1Click(Sender: TObject);
var
   InputString: string;
begin
   PostMessage(Handle,InputBoxMessage,0);
   InputString := InputBox('Input Box','Please Enter a Password','');
end;