Ah, I see, people usually make the mistake of adding the launcher and not the game executable with UE games.
You can go to the game directory, then into the sub-directory with the game's name, Binaries > Win64, and you'll see something like GameName-Win64-Shipping.exe. That is the actual game executable you need to add and configure.
Otherwise, Windows is probably handling the GPU preference.
You can go to Display Settings > Graphics Settings, then select Desktop app and browse to the game executable (not launcher), add it then click on that game in the preference list, go to options, set graphics performance to High performance which should be your dedicated GPU.