I highlighted a few folders in 7zip and wanted to add only the *.cs files from those folders into this archive.
I can't seem to figure out what I would type in the "parameters" box to make this work.

Any help is greatly appreciated.
Well it should be -ir!.\*.cs (though I keep getting a The parameter is not correct. error).
According to "Help" documentation, parameters is only for what you would normally pass to the -m switch, ie the Set Compression Method parameters.
So it seems you can't filter files using the GUI (I seem to recall WinRAR being able to though). You could output a txt file with the relative .cs paths then feed that into the CLI version of 7z, but I imagine that's what you were trying to avoid to begin with.