VOID MarkFullscreenWindow(HWND hWnd, BOOL fFullscreen) Function.

MarkFullscreenWindow(HWND hWnd, BOOL fFullscreen).c

// Marks a window as fullscreen.
VOID MarkFullscreenWindow(HWND hWnd, BOOL fFullscreen)
{
  // Source: https://learn.microsoft.com/en-us/windows/win32/api/shobjidl_core/nf-shobjidl_core-itaskbarlist2-markfullscreenwindow

  ITaskbarList2 *pTaskbarList2 = NULL;
  CoInitializeEx(NULL, COINIT_APARTMENTTHREADED | COINIT_DISABLE_OLE1DDE | COINIT_SPEED_OVER_MEMORY);
  CoCreateInstance(&CLSID_TaskbarList, NULL, CLSCTX_INPROC_SERVER, &IID_ITaskbarList2, (LPVOID *)&pTaskbarList2);

  if (fFullscreen)
    SetPropW(hWnd, L"NonRudeHWND", (HANDLE)FALSE);
  else
    SetPropW(hWnd, L"NonRudeHWND", (HANDLE)TRUE);
  pTaskbarList2->lpVtbl->MarkFullscreenWindow(pTaskbarList2, hWnd, fFullscreen);

  pTaskbarList2->lpVtbl->Release(pTaskbarList2);
  CoUninitialize();
  pTaskbarList2 = NULL;
}