Before anything else you should understand if the game was compiled with Mono or IL2CPP.
If IL2CPP was used the original C# code is now C++ (compiled in native machine code) so you cannot call a C# method at all. If mono was used you should be able to invoke C# method but only if the developer of the game actually put some effort into the support of modding this way (more info here):
http://www.somasim.com/blog/2017/12/tech-notes-unity-game-modding/
Of course there are multiple problems with this, one example is that this is only compatible with windows platform (but one of the awesome thing about Unity is that it will allow cross-platform support with a really small effort (the main thing to make it work on mobile is to create a touchscreen input system)).
Unity games are not easy to mod and the reason is that the framework is made to be cross-platform (and some platform do not support just-in-time and even IL2CPP does not support it). The only real solution is that the developer of the game decide to support a scripting language like lua with something like miniscript or moonsharp, but since you are not the developer I suggest you to contact the developer to understand if he/she will try to implement something to help you with modding.