Use Delphi Dll Wizard to write DLL. 

 Creation: Change the DLL name when saving 

 library test2; 

 uses

  SysUtils,

  Classes,

  forms,

  dialogs;

{$R *.res}

function test():string;

begin

  showmessage('a');

end;

exports
test; // Need to

begin
End. 

 Call: Function test (): string; external 'xxxxxxx \ test2.dll'; 

 Just call the test method in button. 

 Dynamic call DLL: 

 Type
// Night binding, that is, dynamic calling external functions mainly use the following three commands:
// loadlibrary: Get DLL
// getProcaddress: Get function
// FreeLibrary: Release 

   // Define a process type, the parameters should be consistent with the required functions

  TMB = function(hWnd: HWND; lpText, lpCaption: PChar; uType: UINT): Integer; stdcall;

  TForm1 = class(TForm)

    Button1: TButton;

    procedure Button1Click(Sender: TObject);

    procedure FormCreate(Sender: TObject);

    procedure FormDestroy(Sender: TObject);

  private
mb: TMB; {Declaration function mb}
Inst: longword; {Declarier a variable to record the DLL handle to be used}

  public

    { Public declarations }

  end;

procedure TForm1.FormCreate(Sender: TObject);

begin

  inst := LoadLibrary('user32.dll');

  if inst <> 0 then

    MB := GetProcAddress(inst, 'MessageBoxW');

end;

procedure TForm1.FormDestroy(Sender: TObject);

begin
FreeLibrary (Inst); {Remember to release}

end;