Add icon and corresponding resource files to the projects, use MAINICON identifier there to make icon also an application icon, add following three lines of code into form constructor.
HICON icon{ LoadIcon(*reinterpret_cast<const HINSTANCE*>(&Application->Handle), MAKEINTRESOURCE(MAINICON)) };
SendMessage(*reinterpret_cast<const HWND*>(&QueryService()->getWindow()->Handle), WM_SETICON, ICON_BIG, (LPARAM)icon);
SendMessage(*reinterpret_cast<const HWND*>(&QueryService()->getWindow()->Handle), WM_SETICON, ICON_SMALL, (LPARAM)icon);
This code should be called after creating window and renderer. As window and renderer are usually created when calling parent form prototype class constructor, it's Okay to place the code above at the beginning of your form class constructor body.