SendKeys will not work. If you read the documentation (SendKeys.Send(String) Method (System.Windows.Forms) | Microsoft Docs
]) it sends the key to the Active application - which is the app calling SendKeys, not your game.
Since it's a game, they chances are there is little you can do at all - many access the hardware pretty much directly for speed reasons, and Windows messaging is nothing like real time!
And writing a game cheat counts as malicious behavior - it isn't fair on other players.
We do not condone, support, or assist in the production of malicious code in any way, form, or manner. This is a professional site for professional developers.
If you want to know how to create such things, you need to visit a hacking site: but be sure to disable all firewalls and antivirus products first or they won't trust you enough to tell you.