In this example, we are going to use a Form, 2 text box (which the user should enter the both real numbers) and a button. See the Source example:

unit usadll;

interface

uses
Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms, Dialogs, StdCtrls;

type
TForm1 = class(TForm)
Label1: TLabel;
Label2: TLabel;
Button1: TButton;
Edit1: TEdit;
Edit2: TEdit;
procedure Button1Click(Sender: TObject);
private
{ Private declarations }
public
{ Public declarations }
end;
function Max(a, b : double):double;stdcall;

var
Form1: TForm1;

implementation

{$R *.DFM}

function Max(a, b : double):double;external 'MaxDLL';
procedure TForm1.Button1Click(Sender: TObject);
var
x, y, resultado : double;
begin
x := StrToFloat(Edit1.Text);
y := StrToFloat(Edit2.Text);
resultado := Max(x,y);
ShowMessage('Valor Máximo ' +FloatToStr(resultado));
end;

end.

(That's it.. Thank you)