The extension doesn't work when you execute a gamemaker game that was compiled in V2023.8.0.98 and after, but if you use any gamemaker game that was compiled in the past, will work with the extension.
Viewing post in execute_shell_simple for GameMaker comments
This is a known issue on Gamemaker's end.
https://github.com/YoYoGames/GameMaker-Bugs/issues/1673
There are currently two work arounds for this. The first is to use TabularElf's MultiClient.
https://github.com/tabularelf/MultiClient
The second is to compile the game you wish to launch on 2023.6 and execute it in your program.
Apart from that even calling a batch file which launches the game will also produce the same results.