and then find the child windows normally with EnumChildWindow
I'm sorry, I forgot to change the first parameter of ScreenToClient, as you mentioned. Now the same popup menu appears at 100:100 of Desktop window.
This is how I find hMU:
HWND hMU = FindWindow("MU", "Mu Online");
I used Spy++ and my code on the game window, and they both detected same handle value for the game. When I change class name to NULL, it works too. Could the anticheat prevent my attempts to click, probably with a global mouse hook?