This is the simplest kind of a C# WinForms dll injector you can make.
UI Frontend made with my WinForms UI Library CuoreUI - https://github.com/7owh/CuoreUI
You're probably only interested in the Injector.cs class: https://github.com/7owh/basic-dll-injector/blob/master/basic%20dll%20injector/Injector.cs