function InStr(I: Integer): string;
begin
 Str(I, Result);
end;

function InInt(S: string): Integer;
var Code: Integer;
begin
 Val(S, Result, Code);
 if Code<>0 then begin
  Result:=0;
 end;
end;

...
const
 id_edit = 110;
....
CreateWindowEx (0, 'EDIT','',ws_Child or ws_Visible or ws_Border
         or ES_RIGHT,10, 5, 120, 20,  hWindow, id_edit,hInstance, nil);
...

program Project1;

uses
 windows,
 messages,sysutils;

const
 id_Button0 = 100;
 id_edit = 110;



function MainWndProc(hWindow: HWND; Msg: UINT; wParam: wParam;
  lParam: lParam): LRESULT; stdcall; export;
var
 Rect: TRect;
 ps: TPaintStruct;
  s2:string;
  ch: Thandle;
s: PChar;
sl: Integer;
begin
  Result := 0;
  case Msg of
  wm_Create:
   begin
       CreateWindowEx (0,'BUTTON','&0',ws_Child or ws_Visible
       or ws_Border or bs_PushButton,25, 110,45, 20,hWindow,
       id_Button0, hInstance, nil);

       CreateWindowEx (0,'EDIT','',
       ws_Child or ws_Visible or ws_Border or ES_RIGHT,
       10, 5,120, 20,hWindow,id_edit, hInstance,nil);
   end;
   wm_Size:
   begin
     GetClientRect (hWindow, Rect);
    end;
   WM_PAINT:
      begin
       BeginPaint(hWindow, ps);
        SetBkMode(ps.hdc, TRANSPARENT);
        EndPaint(hWindow, ps);
      end;
    WM_DESTROY: begin  freeMem(s);PostQuitMessage(0);end;
    wm_Command:begin
    case LoWord (wParam) of
    id_Button0:
         if HiWord (wParam) = bn_Clicked then  begin
             ch := GetDlgItem (hWindow, id_edit);
             sl := GetWindowTextLength(ch);
             getMem(s, sl+1);
             ZeroMemory(s, sl+1);
             GetWindowText(ch,s,50);
             s2:= s;
             s2:=s2+'0';
             s:=Pchar(s2);
             SetWindowText(GetDlgItem (hWindow, id_edit),s);
             freeMem(s)
             end;
         end;
        end;
    else
      begin
        Result := DefWindowProc(hWindow, Msg, wParam, lParam);
        Exit;
      end;
  end;
end;
var
  wc: TWndClass;
  hWindow: HWND;
  Msg: TMsg;
begin
  wc.lpszClassName := 'YourAppClass';
  wc.lpfnWndProc   := @MainWndProc;
  wc.Style         := CS_VREDRAW or CS_HREDRAW;
  wc.hInstance     := hInstance;
  wc.hIcon         := LoadIcon(0, IDI_APPLICATION);
  wc.hCursor       := LoadCursor(0, IDC_ARROW);
  wc.hbrBackground := (COLOR_WINDOW + 1);
  wc.lpszMenuName  := nil;
  wc.cbClsExtra    := 0;
  wc.cbWndExtra    := 0;
  RegisterClass(wc);
  hWindow := CreateWindowEx(WS_EX_CONTROLPARENT or WS_EX_WINDOWEDGE,
    'YourAppClass',
    'API',
    WS_VISIBLE or WS_CLIPSIBLINGS or
    WS_CLIPCHILDREN or WS_OVERLAPPEDWINDOW,
    CW_USEDEFAULT, CW_USEDEFAULT,
    150, 200 ,
    0,
    0,
    hInstance,
    nil);
  ShowWindow(hWindow, CmdShow);
  UpDateWindow(hWindow);
  while GetMessage(Msg, 0, 0, 0) do
  begin
    TranslateMessage(Msg);
    DispatchMessage(Msg);
  end;
  Halt(Msg.wParam);
end.

GetWindowText(ch,s,50);
SetWindowText(ch,PChar(s+'0'), sl+1);