SetPropW

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

int __stdcall f(HWND Wnd, LPARAM L) {
    SetLastError(ERROR_SUCCESS);
    int Success = SetPropW(Wnd, L"asdf", (HANDLE) 0xdeadbeef);
    int Error = GetLastError();

    wchar_t Class[500] = {'\0'};
    GetClassNameW(Wnd, Class, 500);

    printf("window %x: %S\n", Wnd, Class);
    printf("\tsuccess: %u\n", Success);
    printf("\terror: %u\n", Error);
    printf("\tprop: %x\n\n", (unsigned) GetPropW(Wnd, L"asdf"));
    return 1;
};

void main() {
    EnumDesktopWindows(0, f, 0);
};