switch(iMessage)
{
    case WM_DESTROY:
        PostQuitMessage(0);
        return 0;
    case WM_LBUTTONDOWN:
        MessageBox(hWnd,"테스트 MessageBox입니다.","테스트", MB_OK);
        return 0;
}

int MessageBox(
    HWND hWnd,
    LPCTSTR lpText,
    LPCTSTR lpCaption,
    UINT uType
);