Wäre demjenigen unendlich dankbar!

Müsstest die GetDlgItemInt Funktion zur GetDlgItemTxt umschreiben, und den ausgangswert natürlich auch in einem char speichern.

#define STRICT

#include <windows.h>
#include <iostream>
#include <stdio.h>

LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam);

const char szAppName[]    = "blablabla";

int WINAPI WinMain(  HINSTANCE hInstance, HINSTANCE hPrevInstance,
                     PSTR szCmdLine, int iCmdShow)
{
   MSG        msg;
   HWND       hWnd;
   WNDCLASS   wc;

   wc.style               = CS_HREDRAW | CS_VREDRAW;
   wc.lpfnWndProc         = WndProc;
   wc.cbClsExtra          = 0;
   wc.cbWndExtra          = 0;
   wc.hInstance           = hInstance;
   wc.hCursor             = LoadCursor(NULL, IDC_ARROW);
   wc.hIcon               = LoadIcon(NULL, IDI_APPLICATION);
   wc.hbrBackground       = (HBRUSH) GetStockObject(WHITE_BRUSH);
   wc.lpszClassName       = szAppName;
   wc.lpszMenuName        = NULL;

   RegisterClass(&wc);

   hWnd = CreateWindow(   szAppName,
                          szAppName,
                          WS_OVERLAPPEDWINDOW,
                          CW_USEDEFAULT,
                          CW_USEDEFAULT,
                          CW_USEDEFAULT,
                          CW_USEDEFAULT,
                          NULL,
                          NULL,
                          hInstance,
                          NULL);

   ShowWindow(hWnd, iCmdShow);
   UpdateWindow(hWnd);

   while (GetMessage(&msg, NULL, 0, 0))
   {
      TranslateMessage(&msg);
      DispatchMessage(&msg);
   }
   return msg.wParam;
}

LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam)
{
    static HWND hLogin_User;
    static HWND hButton;
   switch (message)
   {
    case WM_DESTROY:
      {
         PostQuitMessage(0);
         return 0;
      }
    case WM_CREATE:

    hButton = CreateWindow(  "button",
                                  "Beenden",
                                  WS_CHILD | WS_VISIBLE,
                                  550,550, 100, 100,
                                  hWnd,
                                  NULL,
                                  ((LPCREATESTRUCT) lParam) -> hInstance,
                                  NULL);

    hLogin_User = CreateWindowEx(WS_EX_CLIENTEDGE,
    "EDIT",
    "",
    WS_CHILD |
    WS_VISIBLE |
    WS_BORDER
    | ES_NUMBER,
    20,30,200,20,
    hWnd,
    (HMENU) 1,
    (HINSTANCE) GetWindowLong(hWnd, GWL_HINSTANCE),
    NULL);
    return 0;
    break;

    case WM_COMMAND:
        {
            if (lParam == (LPARAM)hButton)
            {
                if (HIWORD(wParam) == BN_CLICKED)
                {
                    BOOL bSuccess;
                    int nTimes = GetDlgItemInt(hWnd, GetDlgCtrlID( hLogin_User), NULL, FALSE);
                    char asdf[30];
                    sprintf(asdf, "%i", nTimes);
                    MessageBox(0, asdf, NULL, MB_OK);
                }
            }
        }
   }

   return DefWindowProc(hWnd, message, wParam, lParam);
}

SendMessage(hLogin_User, EM_LIMITTEXT, 4, NULL);