Process Explorer is your friend. Run the client from the launcher then check the command line for the WoW.exe (WowClassic.exe) process. It looks like the launcher uses a command line of "C:\Games\World of Warcraft\_classic_\WowClassic.exe" -launcherlogin -uid wow_classic in my case. I am guessing it is using a memory map or IPC to transfer a login token to the client. Not easily replicated.