PostQuitMessage(0) won't terminate my 'program'

I'm just trying to learn a little bit of C++ and wrote a few lines that do nothing but open a window. I've added a Message Handler too and by clicking the [X] on my window it closes as its supposed to. As the next step I wanted the Program to terminate when the [X] is clicked but it does'nt do that. Here's my code:

#include <Windows.h>

LRESULT CALLBACK WndProc(HWND hWnd, UINT msg, WPARAM wParam, LPARAM lParam){
    switch (msg){
    case WM_CLOSE:
        PostQuitMessage(88);
        break;
    }

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


int CALLBACK WinMain(HINSTANCE hInstance, HINSTANCE pPrevInstance, LPSTR lpCmdLine, int cCmdShow) {

    const auto pClassName = "M3D";

    WNDCLASSEX wc = { 0 };
    wc.cbSize = sizeof(wc);
    wc.style = CS_OWNDC;
    wc.lpfnWndProc = DefWindowProc;
    wc.cbClsExtra = 0;
    wc.cbWndExtra = 0;
    wc.hInstance = hInstance;
    wc.hIcon = nullptr;
    wc.hCursor = nullptr;
    wc.hIcon = nullptr;
    wc.hCursor = nullptr;
    wc.hbrBackground = nullptr;
    wc.lpszMenuName = nullptr;
    wc.lpszClassName = pClassName;
    RegisterClassEx(&wc);

    HWND hWnd = CreateWindowEx(0, pClassName, "Fenster M3D", WS_CAPTION | WS_MAXIMIZEBOX | WS_SYSMENU, 200, 200, 640, 480, nullptr, nullptr, hInstance, nullptr);

    //Fenster aufrufen
    ShowWindow(hWnd, SW_SHOW);
    //

    //message Pumpe
    MSG msg;
    while (GetMessage(&msg,nullptr,0,0) > 0){
        TranslateMessage(&msg);
        DispatchMessage(&msg);
    }

    return 0;
}

Can anyone tell my whats wrong and why my PostQuitMessage() wont do its job? The other threads I've found werent really helping

The problem is on this assignment:

wc.lpfnWndProc = DefWindowProc;

You are telling the message loop to dispatch messages directly to DefWindowProc(), so your custom WndProc() is never used, so PostQuitMessage() is never called. DefWindowProc() does not call PostQuitMessage() for any message that it processes.

Change that assignment to this instead:

wc.lpfnWndProc = WndProc;

Also, per the Closing the Window documentation on MSDN, DefWindowProc() calls DestroyWindow() when processing WM_CLOSE, so you should call PostQuitMessage() in reply to WM_DESTROY instead:

LRESULT CALLBACK WndProc(HWND hWnd, UINT msg, WPARAM wParam, LPARAM lParam){
    switch (msg){
    case WM_DESTROY:
        PostQuitMessage(88);
        break;
    }

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